adds admin

-added a migration that adds a 'role' attribute to users, this is set to 'user' by default, but we can make ourselves as 'admin' by using the role setter method on ourselves.

-the user's role is added to their session upon login or the creation of a new user

-there are helper methods for stories and pitches that check to make sure that a user is either a) the creator of said pitch or story, or b) and admin before allowing anything to be edited or deleted.