Cross-signing [1/4] -- hidden devices (#5759)

* allow devices to be marked as "hidden"

This is a prerequisite for cross-signing, as it allows us to create other things
that live within the device namespace, so they can be used for signatures.

synapse/storage/schema/delta/56/hidden_devices.sql

@@ -0,0 +1,18 @@
+/* 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.
+ */
+
+-- device list needs to know which ones are "real" devices, and which ones are
+-- just used to avoid collisions
+ALTER TABLE devices ADD COLUMN hidden BOOLEAN DEFAULT FALSE;