Use Action Cable with MessagePack encoding to deliver video stream frames

[ethanjli]

This PR completes part of #141, closes #149, and refactors some internal code.

Breaking changes:

• The TURBOSTREAMS_HASH_KEY environment variable has been renamed to ACTIONCABLE_HASH_KEY (inherited from the upgrade to github/sargassum-world/[email protected])

Additions:

• When Javascript is enabled, the web browser will open camera streams using a new custom HTML element (<video-cable-player>), which opens a new custom Turbo Streams-inspired Action Cable channel (Video::StreamsChannel) over WebSockets, instead of opening the camera stream as MJPEG-over-HTTP. This fixes Turbo Drive continues loading MJPEG stream even after navigating to a different page #149. Video::StreamsChannel integrates with the videostreams Broker in the same way that Turbo::StreamsChannel integrates with the godest framework's turbostreams Broker.
• There is a new /video-cable route which serves Action Cable connections with Video::Streams channels. Each camera stream on an instrument opens a separate WebSocket connection to this route (by attaching a per-camera value on a HTTP GET query param) in order to prevent any camera from being starved from writing data on the WebSocket connection.
• Now all Action Cable connections are served using MessagePack encoding instead of JSON encoding. This allows us to avoid base64-encoding each video stream frame, which would incur a significant network overhead, since JSON can't hold raw bytes.

This PR also upgrades dependencies, both to implement the above additions and to keep dependencies up-to-date.