Actioncable for production

This MR implements 2 changes:

• Actioncable connection is created conditionally based on the Meta tag. It's by default off until logged in.
• No connection limit from nginx side for persistent connections to the cable sub path.

It's not clear whether the separate setting works well with "async" - I think it won't work because this way, there's a new rails instance for the cable stuff. It's here to be merged anyway because I hope it stops the freezing issue. Next step will be either Redis backend or Postgres backend for action cable.