Factor out an "assert_requester_is_admin" function (#5120)

Rather than copying-and-pasting the same four lines hundreds of times

synapse/rest/admin/_base.py

@@ -0,0 +1,59 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from twisted.internet import defer
+
+from synapse.api.errors import AuthError
+
+
+@defer.inlineCallbacks
+def assert_requester_is_admin(auth, request):
+    """Verify that the requester is an admin user
+
+    WARNING: MAKE SURE YOU YIELD ON THE RESULT!
+
+    Args:
+        auth (synapse.api.auth.Auth):
+        request (twisted.web.server.Request): incoming request
+
+    Returns:
+        Deferred
+
+    Raises:
+        AuthError if the requester is not an admin
+    """
+    requester = yield auth.get_user_by_req(request)
+    yield assert_user_is_admin(auth, requester.user)
+
+
+@defer.inlineCallbacks
+def assert_user_is_admin(auth, user_id):
+    """Verify that the given user is an admin user
+
+    WARNING: MAKE SURE YOU YIELD ON THE RESULT!
+
+    Args:
+        auth (synapse.api.auth.Auth):
+        user_id (UserID):
+
+    Returns:
+        Deferred
+
+    Raises:
+        AuthError if the user is not an admin
+    """
+
+    is_admin = yield auth.is_server_admin(user_id)
+    if not is_admin:
+        raise AuthError(403, "You are not a server admin")