create support user (#4141)

Allow for the creation of a support user.

A support user can access the server, join rooms, interact with other users, but does not appear in the user directory nor does it contribute to monthly active user limits.

tests/api/test_auth.py

@@ -50,6 +50,8 @@ class AuthTestCase(unittest.TestCase):
         # this is overridden for the appservice tests
         self.store.get_app_service_by_token = Mock(return_value=None)
 
+        self.store.is_support_user = Mock(return_value=defer.succeed(False))
+
     @defer.inlineCallbacks
     def test_get_user_by_req_user_valid_token(self):
         user_info = {"name": self.test_user, "token_id": "ditto", "device_id": "device"}