[RN] Simplify

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.

react/features/app/functions.web.js

@@ -78,7 +78,7 @@ const _INTERCEPT_COMPONENT_RULES = [
     }
 ];
 
-export { _parseURIString } from './functions.native';
+export { getLocationContextRoot, _parseURIString } from './functions.native';
 
 /**
  * Determines which route is to be rendered in order to depict a specific Redux