* 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
* Add more Broadcasts to seeds for welcome notification flow
* Remove unnecessary welcoming_user_id from development config
* Add basic service class for generating appropriate broadcast per user
Eventually, this should be called from a rake task that will rely on this service to determine which broadcast, if any, is the appropriate one to send to newly signed-up users, and will enqueue a worker to create/send a welcome notification when appropriate.
* Add a few TODOs, plus some general cleanup
* Add some skipped tests for WelcomeNotification::Generator
