Added vestigial DataDeletionServices

Currently can delete a CandidateSession and all associated data;
will move onto other objects that need to be deleted next.
Added delivery deletion to DataDeletionSvc
Filled in remainder of DataDeletionSvc
Filled in svc + MVC for "delete assignment"
Improved flash messages in instructor MVC
Completed "delete delivery" in svc + MVC layers
Merge remote-tracking branch 'origin/master'