Fixes: #312

This reverts commit 890151fa72.

By making the container 100% height and position relative, that
would cause it to overlap any static-positioned  elements below it.
The 100% makes it so that any watermarks intended for the bottom
of the page show up on the bottom of the page. However, it's not
needed because watermark stylings already try to position the
watermarks at the bottom.

* feat(welcome-page): Redesign.

* Style adjustments.

There are (at least) two changes that are breaking:
- defaultTab is gone
- The re-rendering logic looks to have been re-written so that
  passing in a new array of tabs causes a re-render, which can
  reset the currently selected tab.

The fixes involved removing defaultTab from each tab configuration,
as it is no longer respected anyway. Also, instead of letting Tabs
be uncontrolled and allowing it to set its own selected, which
would result in the first tab automatically being selected on
Tabs re-render, use Tabs a controlled prop to dicate which
tab is selected; this is accomplished by specifying a selected
prop.

* Welcome page calendar ui improvements

* Addressing PR review comments.

Calendar integration ui

* feat(welcome-page): be able to open settings dialog

- Create a getter for getting a settings tab's props so the device
  selection tab can get updated available devices.
- Be able to call a function from a tab after it has mounted. This is
  used for device selection to essentially call enumerateDevices on
  the welcome page so the device selectors are populated.
- Remove event UIEvents.AUDIO_OUTPUT_DEVICE_CHANGED. Instead directly call
  setAudioOutputDeviceId where possible.
- Fix initialization of the audioOutputDeviceId in settings by defaulting
  the audio output device to the one set in settings.

* squash: updateAvailableDevices -> getAvailableDevices, add comment for propsUpdateFunction

* fix(welcome-page): css tweaks in prep for branded welcome page

- Watermarks should no longer depend on toolbar size. The left watermark made
  room for the toolbar when the toolbar was on the left side of the screen, but
  the toolbar has been moved to the bottom. The right watermark...well it'll
  clash with the vertical filmstrip but at least the margins will be consistent
  with the left watermark.
- Apply new font-family so fonts are more likely to be consistent across the
  app. Design likes SF UI and keeps requesting it so use it by default.
- Change sizings of welcome page header to be more responsive. This will help
  the header be scrollable when there is no additional content and the header
  overflows.
- Change colors of the welcome page header and remove background image that
  was in the header. Leave in the dom for the background image in case other
  deployments need to continue showing an image.
- Add a period to the title of the welcome page.
- Move watermarks dom location as it is not part of the header; it's part of the
  whole page.

* [squash] Size and font adjustments. Renaming.

In order to be able to add analytics to the deep-linking pages the
lib-jitsi-meet initialization has been moved so it happens earlier.

The introduced `initPromise` will eventually disappear, once conference is
migrated into React and / or support for Temasys is dropped. At that stage, it
can be turned into a sync function which all platforms share.

* feat(Deeplinking): Implement for web.

* ref(unsupported_browser): Move the mobile version to deeplinking feature

* feat(deeplinking_mobile): Redesign.

* fix(deeplinking): Use interface.NATIVE_APP_NAME.

* feat(dial_in_summary): Add the PIN to the number link.

* fix(deep_linking): Handle use case when there isn't deep linking image.

* fix(deep_linking): css

* fix(deep_linking): deeplink -> "deep linking"

* fix(deeplinking_css): Remove position: fixed

* docs(deeplinking): Add comment for the openWebApp action.

* fix(welcome-page): modify styling for narrow screens

* squash: fix autoscrolling on mobile safari

* feat(welcome): new design

* squash: update strings

* squash: copy/paste error?

* squash: remove welcome page disabling checks

* squash: change strings again

* squash: background var

* squash: title and desc css as variables

* Javadoc introduced @code as a replacement of <code> and <tt> which is
  better aligned with other javadoc tags such as @link. Use it in the
  Java source code. If we switch to Kotlin, then we'll definitely use
  Markdown.

* There are more uses of @code in the JavaScript source code than <tt>
  so use @code for the sake of consistency. Eventually, I'd rather we
  switch to Markdown because it's easier on my eyes.

* Xcode is plain confused by @code and @link. The Internet says that
  Xcode supports the backquote character to denote the beginning and end
  of a string of characters which should be formatted for display as
  code but it doesn't work for me. <tt> is not rendered at all. So use
  the backquote which is rendered itself. Hopefully, if we switch to
  Markdown, then it'll be common between JavaScript and Objective-C
  source code.

It will replace the "Join" text button while appNavigate lasts.

Note about the implementation: when appNavigate completes the component
may have been unmounted and thus we cannot touch its state. In order to
avoid this problem I added a 'mounted' instance variable which gets set
and reset in componentWillMount / Unmount respectively. This is to avoid
using isMounted, which is highly discouraged.

jsdocs were added, remove old comment which no longer applies.

Split language detectors to be web/native dependent. Take care of strings that contain html.

When rendering using a for loop each child whould have a key prop.

Until we make a decision on access modifier hints and adopt a respective
coding style, consistency is king.

Recently, we reimplemented the Welcome page in React. Unfortunately, we
broke the checkbox that enables/disables the Welcome page and it would
allow checking but wouldn't allow unchecking.

Recently, we reimplemented the watermarks in React. Unfortunately, we
didn't take into account film strip-only mode.

Additionally, we duplicated watermark-related source code on the Welcome
and Conference pages.

The React-based rewrite looks whether there's a room name (in the
window's location) in order to choose between WelcomePage and
Conference. But app.js expects Conference to be rendered before it
builds a room name if WelcomePage is disabled and there's no room name.
A quick and dirty workaround is to render Conference within WelcomePage
so that the rendered result closely resembles index.html before the
React-based rewrite.

As a step toward merging jitsi-meet-react with jitsi-meet to share as
much source code as possible between mobile and Web, merge the part of
jitsi-meet-react's source tree which supports mobile inside the
jitsi-meet source tree and leave jitsi-meet-react's Web support in the
source code revision history but don't have it in master anymore because
it's different from jitsi-meet's Web support. In other words, the two
projects are mechanically merged at the file level and don't really
share source code between mobile and Web.