A follow-up to #15117 and #15141. Applies the previous changes to PM-specific fields, makes the preview area take the all the available height of the composer, and unifies more spacing between composer elements.
Previously the discourse-presence plugin was using a `position: absolute` hack to display the 'replying...' users in the top right of the composer. This commit adds a more suitable plugin outlet, and updates the discourse-presence styling so it slots into the flex-box layout at the top of the composer
This removes all custom controllers and redis/messagebus logic from discourse-presence, and replaces it with core's new PresenceChannel system.
All functionality should be retained. This implementation should scale much better to large numbers of users, reduce the number of HTTP requests made by clients, and reduce the volume of messages on the MessageBus.
For more information on PresenceChannel, see 31db8352
