Speech bubble waiting for an ajax response prior to rendering

I've tried to address speech bubble ajax response problem, this PR does following changes:

* On clicking speech bubble, the ajax request will be made if any of the following condition is true:
  * User has unread/new notifications.
  * User has unread/new private messages.
  * User clicks on speech bubble for the first time, and `notifications` are empty/null.
* If any of the above condition is not true, then no ajax request will be made, thus improving notification render significantly. 

**TO-DO**:

* Find a better way to detect that user clicked on speech bubble for the first time, instead of `!self.get("notifications")`. 
* Show drop-down immediately with spinning icon when ajax request is made.

PS, I am just getting started with Ember, please let me know if there's a better way to do this.