Admin/Staff Users should add a list of Github usernames that can authenticate to the application

[halfghaninne]

No others should be allowed to automatically authenticate.

ORIGINAL PLAN HIDDEN

Maybe this happens like so: - Staff Users should have the ability to create Student Users - In the User model there should be some utility method `check_user_acess` - `check_user_access` should be called in a conditional in the `self.from_omniauth(auth_hash)` method in the User model.

[REVISED 9/2/16]

• Staff Users should have the ability to create Student Users

  Streamlining this process should be its own Issue, I think

• [x] In the User model there should be some Class method check_user_access
• [x] check_user_access should be called in a conditional in sessions_controller.rb

[halfghaninne]

Notes to self:

- Admin User creates a cohort - Admin User creates Student Users that belong to cohort - Students belong to cohorts through profiles. Therefore, Admin User must also create these. > A note for another time, maybe if a User is a Student they MUST have a cohort? Probably no way to do conditional validations on a single model like that - Student Users can only log into the application if they already exist in the application --- > A consideration after this amount of work is finished -- what happens if a student changes their email or github username?