* 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.

As an intermediate step on the path to merging jitsi-meet and
jitsi-meet-react, import the whole source code of jitsi-meet-react as it
stands at
2f23d98424
i.e. the lastest master at the time of this import. No modifications are
applied to the imported source code in order to preserve a complete
snapshot of it in the repository of jitsi-meet and, thus, facilitate
comparison later on. Consequently, the source code of jitsi-meet and/or
jitsi-meet-react may not work. For example, jitsi-meet's jshint may be
unable to parse jitsi-meet-react's source code.