Staff landing page

* Add staff/base controller
* set cohorts instance variable to pass to index view
* Add placeholder text to staff dashboard landing page
* Add enum to user model for roles
* Add role, first/last name fields to user model/table
* Update seed data with names and one user per role
* Update initial check in site controller to redirect user based on role
* Update nav partial to display current_user first_name instead of "Student"
* Add proper route for staff_path