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.

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).

* feat(tile-view): initial implementation for mobile

- Create a tile view component for displaying thumbnails in a
  two-dimensional grid.
- Update the existing TileViewButton so it shows a label in the
  overflow menu.
- Modify conference so it can display TileView while hiding
  Filmstrip.
- Modify Thumbnail so its width/height can be set and to prevent
  pinning while in tile view mode.

* use style array for thumbnail styles

* change ternary to math.min for expressiveness

* use dimensiondetector

* pass explicit disableTint prop

* use makeAspectRatioAware instead of aspectRatio prop

* update docs

* fix docs again (fix laziest copy/paste job I've ever done)

* large-video: rename onPress prop to onClick

* change forEach to for...of

* use truthy check fallthrough logic instead of explicit if

* put tile view button second to last in menu

* move spacer to a constant

* the magical incantation to make flow shut up

Moves NotificationContainer to the toolbox and filmstrip container, so
that there's no need to manually calculate the positions.

The current code was splitting it on a space, which made nicknames such as “Link Mauve” appear as “Link”, whereas it gets displayed correctly everywhere else in the UI.

* feat(tile-view): initial implementation for tile view

- Modify the classname on the app root so layout can adjust
  depending on the desired layout mode--vertical filmstrip,
  horizontal filmstrip, and tile view.
- Create a button for toggling tile view.
- Add a StateListenerRegistry to automatically update the
  selected participant and max receiver frame height on tile
  view toggle.
- Rezise thumbnails when switching in and out of tile view.
- Move the local video when switching in and out of tile view.
- Update reactified pieces of thumbnails when switching in and
  out of tile view.
- Cap the max receiver video quality in tile view based on tile
  size.
- Use CSS to hide UI components that should not display in tile
  view.
- Signal follow me changes.

* change local video id for tests

* change approach: leverage more css

* squash: fix some formatting

* squash: prevent pinning, hide pin border in tile view

* squash: change logic for maxReceiverQuality due to sidestepping resizing logic

* squash: fix typo, columns configurable, remove unused constants

* squash: resize with js again

* squash: use yana's math for calculating tile size

Which key should we use? Using "L" for now.