* add button state / helper dialog

* new audio screen share flow

* change error message

* address a couple of merge conflicts

* fix lint

* address code review

* address code review

* restrict audio screen share on mobile browsers

Through several avatar work iterations it's no longer used.

Up until now we relied on implicit loading of middlewares and reducers, through
having imports in each feature's index.js.

This leads to many complex import cycles which result in (sometimes) hard to fix
bugs in addition to (often) breaking mobile because a web-only feature gets
imported on mobile too, thanks to the implicit loading.

This PR changes that to make the process explicit. Both middlewares and reducers
are imported in a single place, the app entrypoint. They have been divided into
3 categories: any, web and native, which represent each of the platforms
  respectively.

Ideally no feature should have an index.js exporting actions, action types and
components, but that's a larger ordeal, so this is just the first step in
getting there. In order to both set example and avoid large cycles the app
feature has been refactored to not have an idex.js itself.

* Add a checkbox for skipping the prejoin page on next use. (This is hidden for
now, until we also have a settings entry for it).
* Rework 'Join by Phone' buttons and add new overlay.
* Update the device status accordingly if there were errors while adding
devices.
* The input is filled with the display name if there was one previously used.
* Join the meeting on 'Enter' press.

Currently only 2 options are implemented, mainly aimed at helping troubleshoot
audio related problems:

- Disable native call integration (it disables CallKit / ConnectionService)
- Disable P2P

React Native doesn't define __filename nor __dirname so do it artisanally. In
addition, this helps with centralizing the configuration passed to loggers.

If a value is not set in localStorage then null is
returned. null should not be converted to an empty
string (via _.escape) because that will then be
stored in localStorage as the user set preference
and will keep overriding any other values set
in localStorage for the displayname.

* Skips setting undefined device id to sink in audio preview.

* Fallbacks to use labels for user selected devices.

* Fixes comment.

The state about currently opened devices is filtered and not stored, where we only store when user selects a device preferences.
Also allow changing input devices for Firefox when we are not in a conference.

When the iFrame api is used to set a preferred audio output using
options passed into the JitsiMeetExternalAPI constructor, no logic
fires to actually change the audio output destination.

* 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

BaseApp does all the heavy-lifting related to creating the redux store,
navigation, and so on.

App currently handles URL props and actually triggering navigation based on
them.