invitations: Send 'invites_changed' event for invitations events.

Fixes #7665

In case of invitation events, 'invites_changed' event without
any real payload is sent to all the realm admins and the user.
The event is handled by reloading the list to view recent changes.

Commit tweaked by shubhamdhama:
* Send an `invite_changed` event when an user accept an invite.
    Also, added the test for the same.
* No need to delete the invite list in frontend, current logic
    handles the case when the invite data is changed properly.
* Extracted the common logic for sending an event into
    `notify_invites_changed`.

zerver/lib/events.py

@@ -607,6 +607,8 @@ def apply_event(state: Dict[str, Any],
     elif event['type'] == "update_global_notifications":
         assert event['notification_name'] in UserProfile.notification_setting_types
         state[event['notification_name']] = event['setting']
+    elif event['type'] == "invites_changed":
+        pass
     elif event['type'] == "user_group":
         if event['op'] == 'add':
             state['realm_user_groups'].append(event['group'])