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.