CONN-464: Added 60s caching on Notif::getUpdateCounts; Switched to GET;

This pull request deals with TopNav Notifications caching
* The **WallNotificationsExternal::getUpdateCounts** call gets cached for 60 seconds in the user browser;
* Removed the **username: wgTitle** param from the request as it is not used and also doesn't make much sense except as a cache-buster, which we're trying to prevent;
* Added conditional notification collection for logged users only to prevent potentially expensive and not required operation in case the user session has expired, but the user hasn't refreshed the page yet.

@macbre can you please take a look if the code is OK and if the caching interval is sufficient