Refactor SynchronizedEditorChannel connect / confirmation

[kiragrammel]

In production, it takes some time for users to be actually connected with a channel through ActionCable. This causes problems, since the synchronized editor relies on an immediate connection to work. Otherwise, the text written by another learner might not get synchronized, and also the online status is not shown correctly.

As a first countermeasure, the ActionCable method ensure_confirmation_sent has been overwritten in 2c46ad2 (channels/application_cable/channel.rb), which is not ideal.

For the specific case of the synchronized editor, we might be able to refactor the code by using the connected() method in assets/javascripts/channels/synchronized_editor_channel.js.