It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
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
