Track IS used to bind 3PIDs

This will then be used to know which IS to default to when unbinding the
threepid.

synapse/storage/schema/delta/53/user_threepid_id.sql

@@ -0,0 +1,27 @@
+/* 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.
+ */
+
+-- Tracks when
+CREATE TABlE user_threepid_id_server (
+    user_id TEXT NOT NULL,
+    medium TEXT NOT NULL,
+    address TEXT NOT NULL,
+    id_server TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX user_threepid_id_server_idx ON user_threepid_id_server(
+    user_id, medium, address, id_server
+);
+