* Create EmailDigest WIP
* Add email_digest_periodic to User model
* Add email digest setting to notification
* Draft thoughts (?) WIP
* Implement EmailDigest's features
* Remove hard-coded email digest vars
* Create spec for EmailDigest
* Fix something
* Temp work
* Run migration & Yarn
* Move email logic out of EmailDigest
* Improve email logic
* Improve EmailLogic'specs
* Update copy
* Change positive_reactions_count limit
For non-followed articles we should have a higher limit. So this change modifies that.
