Implements interface on_logged_out so that synapse can run check_3pid_auth

Author: anishihara

rest_auth_provider.py

@@ -53,6 +53,8 @@ async def check_3pid_auth(self, medium, address, password):
             return None
         return address
 
+    async def on_logged_out(self, user_id, device_id, access_token):
+        return True
 
     async def check_password(self, user_id, password):
         logger.info("Got password check for " + user_id)