GET /devices endpoint

implement a GET /devices endpoint which lists all of the user's devices.

It also returns the last IP where we saw that device, so there is some dancing
to fish that out of the user_ips table.

synapse/storage/schema/delta/33/user_ips_index.sql

@@ -0,0 +1,16 @@
+/* Copyright 2016 OpenMarket 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.
+ */
+
+CREATE INDEX user_ips_device_id ON user_ips(user_id, device_id, last_seen);