Restricting access for creating, editing, destroying agencies & foodtypes

*add a column in users table that describes the role of the user
*this column is an enum, and has value of donor, agency_admin, super_admin
*added an admin controller that includes the before_action
*namespaced admin controllers and views, all inheriting from admin controller
Merge pull request #3 from katherinlaine/ks-restrict-access-to-admin-dash

Restricting access for creating, editing, destroying agencies & foodtype