* 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
