* Add Welcome Thread Broadcast to generator.rb
* Add welcome trait to broadcasts.rb
* Add additional tests around welcome_broadcasts:
- ensure that the correct Broadcast is sent
- ensure that a User only receives a single Notification
- ensure that only certain Users receieve the Notification
* Refactor and remove unncessary code from generator.rb and generator_spec.rb
* Refactor generator_spec and eagerly load welcome_thread_comment to get spec passing
* Initialize user in place of receiver_id in generator.rb
* Add before action to make generator_spec more readable
* Add latest_published_thread method to generator.rb
* Update generator.rb to be reusable with latest_published_thread
* Update generator_spec to use welcome tags for Article obj
* Create mascot_account using let to reduce User creation in spec
* Adjust expectation for a User who should not receive a notification
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