Prepare for React 16.

- Re-use the native redux pinning implementation for web
- Remove pinning logic from conference.js
- To the native pinning add a check for sharedVideo so
  youtube videos do not send a pin event
- Add shared videos as a participant to enable pinning and
  so they can eventually get added to the filmstrip
- Emit UIEvents.PINNED_ENDPOINT from middleware

Rewrite the non-React RingOverlay into the React Component CallOverlay
in a way which makes it easier to port to React Native.

The counterpart of the external API in the Jitsi Meet Web app uses the
search URL param jwt to heuristically detect that the Web app is very
likely embedded (as an iframe) and, consequently, needs to forcefully
enable itself. It was looking at whether there was a JSON Web Token
(JWT) but that logic got broken when the JWT support was rewritten
because the check started happening before the search URL param jwt was
parsed.

There were getDomain, setDomain, SET_DOMAIN, setRoomURL, SET_ROOM_URL
which together were repeating one and the same information and in the
case of the 'room URL' abstraction was not 100% accurate because it
would exist even when there was no room. Replace them all with a
'location URL' abstraction which exists with or without a room.

Then the 'room URL' abstraction was not used in (mobile) feature
share-room. Use the 'location URL' there now.

Finally, removes source code duplication in supporting the Web
application context root.