[META] Port Chatroom for Drupal 8

Let's track issues needed to release a stable chatroom module for D8 here.

TODO

• #2640492: remove Drupal\chatroom\Chatroom class, or renovate it
• #2277997: [META] Port nodejs for Drupal 8.
• #2640494: creating a chatroom fails with "Unknown column 'title' in 'field list'"

Done

• #2630490: Turn chatrooms and messages into entities