Add cross-signing:

- Add properties to the response of `GET /user/keys` and `GET
  /user/devices/{userId}`.
- The `m.device_list_update` EDU is sent when a device gets a new signature.
- A new `m.signing_key_update` EDU is sent when a user's cross-signing keys
  are changed.

per [MSC1756](https://github.com/matrix-org/matrix-doc/pull/1756).