This is an engine living in action_cable/engine.rb, convention is to
call these things *::Engine.
Looking at thi git history looks like the current *::Railtie was just
an accident.
Instead of depending on ApplicationCable::Connection being defined at initialize
we should inject it in the Railtie.
Thus we can kill more setup in the tests too.
We were explicitly referencing Rails.root in ActionCable::Server::Configuration.initialize,
thereby coupling ourselves to Rails.
Instead add `app/channels` to Rails' app paths and assign the existent files
to `channel_paths`.
Users can still append to those load paths with `<<` and `push` in `config/application.rb`.
This means we can remove the custom `Dir` lookup in `channel_paths` and the Rails
and root definitions in the tests.
The application can have more than one ApplicationController and we
want Action Cable helpers in all controllers and their views so we
should hook in the same place that others helpers.
