add manager as a role

[annkatrinkuessner]

A manager role should be added that reflects the differences between a "Schülerklubsprecher" (organizer) and a "ÖA Mitarbeiter" (from now on manager) in our use case.
A manager works in administration and helps with planning workshops and camps.
A manager should be able to:

• can [:show, :index, :archive], Event
• can [:view_participants, :view_material, :upload_material, :download_material, :print_agreement_letters, :view_unpublished, ], Event
• can [:show, :index], Request
• can :send_email, Email
• cannot :view_apply_button, Event
• cannot :check, ApplicationLetter
• cannot :view_apply_button, Event
• cannot [:edit, :update, :destroy], Event
• cannot [:update], ParticipantGroup
• cannot :update_role,
• cannot :update_role_to_admin, User

Depends on #672.