This commit adds a keyboard shortcut (Shift+ESC) for chat which marks all
of the chat channels that the user is currently a following member of as read,
updating their `last_read_message_id`. This is done via a new service.
It also includes some refactors and controller changes:
* The old mark message read route from `ChatController` is now supplanted
by the `Chat::Api::ReadsController#update` route.
* The new controller can handle either marking a single or all messages read,
and uses the correct service based on the route and params.
* The `UpdateUserLastRead` service is now used (it wasn't before), and has been slightly
updated to just use the guardian user ID.
a0381157e9