Create a Repository Manager role

Create a 'manager' method in base.rb that reads a list of manager
email addresses from config/manager_usernames.yml. Modify ability.rb
to check that the current user is a manager and grant
edit/update/destroy access on everything except users/profiles.
Modify curate_generator.rb to create a default yaml file.
(Editing other users and making everything discoverable will be done
in future stories)

HYDRASIR-201 #close
HYDRASIR-202 #close
Merge pull request #421 from projecthydra/feature/HYDRASIR-95-create-repository-manager-role

Create a Repository Manager role