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/devices.py

@@ -65,7 +65,7 @@ class DeviceStore(SQLBaseStore):
             user_id (str): The ID of the user which owns the device
             device_id (str): The ID of the device to retrieve
         Returns:
-            defer.Deferred for a namedtuple containing the device information
+            defer.Deferred for a dict containing the device information
         Raises:
             StoreError: if the device is not found
         """
@@ -75,3 +75,23 @@ class DeviceStore(SQLBaseStore):
             retcols=("user_id", "device_id", "display_name"),
             desc="get_device",
         )
+
+    @defer.inlineCallbacks
+    def get_devices_by_user(self, user_id):
+        """Retrieve all of a user's registered devices.
+
+        Args:
+            user_id (str):
+        Returns:
+            defer.Deferred: resolves to a dict from device_id to a dict
+            containing "device_id", "user_id" and "display_name" for each
+            device.
+        """
+        devices = yield self._simple_select_list(
+            table="devices",
+            keyvalues={"user_id": user_id},
+            retcols=("user_id", "device_id", "display_name"),
+            desc="get_devices_by_user"
+        )
+
+        defer.returnValue({d["device_id"]: d for d in devices})