Add access control for digital objects
Add access control for pdf download
Add functions expired? and DngSession.access_control_key
access_control_key now calls DngSession.access_control_key
Use access_control_key
Use access_control mechanism and params[:ac]
Implement access_rights
Add has_many :dng_sessions
Add :sc label to FILENAME_METADATA_TAGS
Add belongs_to :access_right