Add `role_required` view decorator to check current_user role
Add `role_required` view decorator to check current_user role

Returns 403 if user has none of the roles listed in the decorator
arguments.

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend