[ALMOST READY] ensure membership request approved notification renders properly

Rendering the membership_request_approved notification was causing a javascript error(silent due to the rescue), the result was that the notification does not show and the notification count stays on one and wont dismiss.
This was due to the approver not being set in the Event and the membership_request_approved logic missing from the NotificationItem constructor.
The user_added_to_group event was also storing the wrong inviter on the Event which is now fixed by this PR.

No cucumber specs have been written as I don't have time. But the unit tests have been updated and should protect from any mistaken changes in the code. The bug is fixed.

@HSalmon , @jdoud , @s01us  QA?