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.

🔥🔥🔥

Using anything non-serializable for action types is discouraged:
https://redux.js.org/faq/actions#actions

In fact, this is the Flow definition for dispatching actions:

declare export type DispatchAPI<A> = (action: A) => A;
declare export type Dispatch<A: { type: $Subtype<string> }> = DispatchAPI<A>;

Note how the `type` field is defined as a subtype of string, which Symbol isn’t.

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