Regresssion from bd8a7edbd2.

When the toolbar buttons are overridden with URL parameters, our computed set of
buttons will be wrong. Thus, compute it every time and check for the
differences.

- Adds the ability to share video as a "PiP" when screenshare is in progress.
- Add a method for creating a local presenter track.
- Make sure isLocalVideoTrackMuted returns the correct mute state when only screenshare is present.
- Make sure we get the updated window size of the window being shared before painting it on the canvas.
- Make sure we check if the shared window has been resized

* Add shortcut key for toggle tile view

* Toggle tile view shortcut - undo main-enGB.json

* Add analytics

* Use already defined toolbar translations

- simplify initialization procedure
- set user display name as the Etherpad name\
- use SharedDocumentButton

This refactors all handling of audio-only and last N to 2 features in preparation
for "low bandwidth mode".

The main motivation to do this is that lastN is a "global" setting so it helps
to have all processing for it in a single place.

* Updates start silent, turning on startWithAudioMuted and few UI tweaks.

Disabled mic unmute button and removes remote participants volume slider.

* Adds analytics for start silent.

* Removes extra semi colon.

* Updates lib-jitsi-meet and updates meeting info text.

When the video unmute button disabled audio-only, also unmute video. This fixes
a weird case in which the user need to "unmute twice" if they were muted beofre
they enabled audio-only mode. That's ok if the audio-only button was used, but
not if the video-unmute button was used, since the expectation is to have video,
of course.

If audio-only was engaged, use the video (un)mute button to disengage it. This
should simplify the flow for getting back to video.

This PR changes the logic for connecting / disconnecting conferences. Instead of
doing it in mount / unmount events from the Conference component, it moves the
logic to the appNavigatee action.

This fixes a regression introduced in 774c5ecd when trying to make sure the
conference terminated event is always sent.

By moving the logic to appNavigate we no longer depend on side-effects for
connecting / disconnecting, and the code should be more maintainable moving
forward.

An improvement to this is the concept of sessions, which, while not tackled
here, was taken into consideration.

🔥🔥🔥

While Apple doesn't want to allow us to enable Dropbox, it's good to have it
available for testing.

Co-authored-by: DimaG <dgeorgiev06@gmail.com>

Apple rejected our app on account of requiring Dropbox not being acceptable. Oh
well! Disable it until we find a way around it. Sigh.

For the most part the changes are taking the "static propTypes" declaration off
of components and declaring them as Flow types. Sometimes to support flow some
method signatures had to be added. There are some exceptions in which more had
to be done to tame the beast:
- AbstractVideoTrack: put in additional truthy checks for videoTrack.
- Video: add truthy checks for the _videoElement ref.
- shouldRenderVideoTrack function: Some component could pass null for the
  videoTrack argument and Flow wanted that called out explicitly.
- DisplayName: Add a truthy check for the input ref before acting on it.
- NumbersList: Move array checks inline for Flow to comprehend array methods
  could be called. Add type checks in the Object.entries loop as the value is
  assumed to be a mixed type by Flow.
- AbstractToolbarButton: add additional truthy check for passed in type.

- Change "features/chat" to support listening for new chat messages
  and storing them, removing that logic from conference.js.
- Combine chat.scss and side_toolbar_container.css, and remove unused
  scss files. Chat is the only side panel so the two concepts have
  been merged.
- Remove direct access to the chat feature from non-react and non-redux
  flows.
- Modify the i18n translate function to take in an options object.
  By default the option "wait" is set to true, but that causes
  components to mount after the parent has been notified of
  an update, which means autoscrolling down to the latest rendered
  messages does not work. With "wait" set to false, the children
  will mount and then the parent will trigger componentDidUpdate.
- Create react components for chat. Chat is the side panel
  plus the entiren chat feature. ChatInput is a child of Chat and
  is used for composing messages. ChatMessage displays one message
  and extends PureComponent to limit re-renders.
- Fix a bug where the toolbar was not showing automatically when
  chat is closed and a new message is received.
- Import react-transition-group to time the animation of the
  side panel showing/hiding and unmounting the Chat component.
  This gets around the issue of having to control autofocus if the
  component were always mounted and visibility toggled, but
  introduces not being able to store previous scroll state
  (without additional work or re-work).