While the base URL remains configurable, this patch reverts back to using
Gravatar.

We noticed high latency with libravatar and contacted them. They are in the
process of migrarting to a better infrastructure (it's a single personal server
at the moment) so we'll re-evaluate once that has happened.

As for why not leave the default and change it on the meet.jit.si installation,
we don't want to kill their server :-)

Also, default to libravatar.

Closes: https://github.com/jitsi/jitsi-meet/issues/4927

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

Use fetch(), which gives us better control over headers.

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.

getLocalParticipant returns a participant object stored in Redux, not a
JitsiParticipant object.

* Add participantRoleChanged event to external API

* Update API documentation

* Remove unneeded whitespace

* Remove more unneeded whitespace

* Fix jsdoc formatting

* Fix jsdoc formatting

Refs: https://github.com/jitsi/lib-jitsi-meet/pull/1108

It's been considered too disruptive and will often misfire especially
if there are issues with the data channels.

It's an evolution of audio-only mode, where we also allow for receiving a remote
screen-share.

Diving deeper: this basically sets last N to 1 or 0 depending on the
availability of a screen-share.

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.

Otherwise the participant will not have been added yet
to state so the participant name will not display in
the notification.

Also, use the helper function to get the display name for a participant.

* Updates kick showing who kicked us.

* Notify participants that someone was kicked.

* Shows notification to user who is remotely muted.

* Updates the notification type.

* Muted by notification for mobile.

* Moves code to react and adds the kick notifications to mobile.

* Updates lib-jitsi-meet.

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.