feat(config.js): add 'websocket' config option

Config.js will allow to specify both BOSH and Websocket URLs. In such
case the web app will prefer Websocket over BOSH. The reason is that it
appears to be more stable and a bit fast on web, while on mobile
websocket is dropped fast(killed by the OS) on network changes.

react/features/base/connection/actions.native.js

@@ -312,7 +312,8 @@ function _constructOptions(state) {
 
         room && (bosh += `?room=${getBackendSafeRoomName(room)}`);
 
-        options.bosh = bosh;
+        // FIXME Remove deprecated 'bosh' option assignment at some point.
+        options.serviceUrl = options.bosh = bosh;
     }
 
     return options;