Ensure that the session is logged out server side, not just client side.

Anytime a user clicks "sign out" we should be signing them out server side as well. This can be accomplished by setting the Users.current_session_id = null.
I found that the method User.logged_in_elsewhere doesn't need to check if the current_session_id is None. The current_session_ids in the cookie and db (redis or postgres) then the user should be forced to log in again.

app/notify_client/user_api_client.py

@@ -10,7 +10,8 @@ ALLOWED_ATTRIBUTES = {
     'email_address',
     'mobile_number',
     'auth_type',
-    'updated_by'
+    'updated_by',
+    'current_session_id'
 }