* Add migrations for subscriber_email column
* Add new schema
* Update relationships, specs, and factory
* Use unless index_exists? over if !index_exists?
* Fix typo in last fix for migration
* Rename authored_user_subscriptions
- to source_authored_user_subcriptions
* Add factory methods and specs
- UserSubscription.make
- UserSubscription.make_new
- Article.new_user_subscription
- Article.create_user_subscription
* Remove self and &
* Change make_new to build
* new_user_subscription --> build_user_subscription
* Come on, Travis
* Backend MVP
- Migration
- Model
- Associations
* Add factory, spec, and validations
- Add unqiue index for validations
* Fix specs
* Add author to email_subscriptions
* Add user association to set_author_id
* Add clarifying comment to set_author_id
* Writing is hard sometimes
* Rename email_subscriptions to subscription_sources
* UserSubscription and user_subscription_sourceable
* subscribers --> sourced_subscribers
* Add comment to explain UserSubscription model
