fix(action_cable): add missing ApplicationCable::Channel base class

- MozoChannel < ApplicationCable::Channel was failing with
  NameError: uninitialized constant ApplicationCable::Channel
- Standard Rails convention requires both connection.rb and channel.rb

app/channels/application_cable/channel.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+module ApplicationCable
+  class Channel < ActionCable::Channel::Base
+  end
+end