[feature] Added a WebSocket endpoint for broadcasting all location updates #191

- Added a new Channels consumer at ``/ws/loci/location/all/`` to
  broadcast updates for all locations in a single feed.
- Implemented ``BaseCommonLocationBroadcast`` and ``CommonLocationBroadcast``.
- Updated signal receivers to publish to both per-location and
  common broadcast groups.
- Switched ``CHANNEL_LAYERS`` in the test project to use Redis.
- Updated local tooling and CI to include Redis service.

Closes #191