Add FactoryGirl factories for artist and user

Seconodary testing helper for situations shuolda.
Add user model and database migration

Has many favorites
Has many artists through favorites
Has many events through artists
Has many notifications
Add event model and database migration

Event has many artists
Event has many notifications
Add favorite model and database migration

- Join table between users and artists
Add notification model and database migration

Belongs to user and event
Fix user validation arguments

Artist validations erroneously included in user model.
Rename feature test for adding user favorites