fix(presence): send latest presence on conference join (#920)

Attempts to fix the following issue:
1. Electron user starts the conference join process.
2. Electron user adds remote control capability and
   attempts to send presence.
3. Electron user presence send is blocked because the
   conference has not been joined.
4. Electron user joins the conference.
5. The remote control capability is not emitted until
   some other presence change triggers another
   presence send. Until then, other uses will not see
   remote control option available for the electron
   user.

modules/xmpp/ChatRoom.js

@@ -460,6 +460,10 @@ export default class ChatRoom extends Listenable {
                     this.locked = true;
                 }
 
+                // Re-send presence in case any presence updates were added,
+                // but blocked from sending, during the join process.
+                this.sendPresence();
+
                 this.eventEmitter.emit(XMPPEvents.MUC_JOINED);
             }
         } else if (this.members[from] === undefined) {