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/handlers/device.py

@@ -69,3 +69,30 @@ class DeviceHandler(BaseHandler):
                 attempts += 1
 
         raise StoreError(500, "Couldn't generate a device ID.")
+
+    @defer.inlineCallbacks
+    def get_devices_by_user(self, user_id):
+        """
+        Retrieve the given user's devices
+
+        Args:
+            user_id (str):
+        Returns:
+            defer.Deferred: dict[str, dict[str, X]]: map from device_id to
+            info on the device
+        """
+
+        devices = yield self.store.get_devices_by_user(user_id)
+
+        ips = yield self.store.get_last_client_ip_by_device(
+            devices=((user_id, device_id) for device_id in devices.keys())
+        )
+
+        for device_id in devices.keys():
+            ip = ips.get((user_id, device_id), {})
+            devices[device_id].update({
+                "last_seen_ts": ip.get("last_seen"),
+                "last_seen_ip": ip.get("ip"),
+            })
+
+        defer.returnValue(devices)