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
Fixed wrong HTML titles in these JSPs
Merge remote-tracking branch 'origin/master'
Removed extraneous entry in CSV header
Fixed NPE in TSController.computeItemSubset()

This was caused by lack of clarity in what could be the
result of TestPlan.getNodes(). I've improved the docs to clarify this.
Fixed regression in LTI auth filter

(I failed to check some of the launch parameters to make
sure they are not null.)
Partial implementation of 'delete del settings'

This is implemented as far as the service layer,
but not the web layer as yet, as I need to decide
how best to handle settings that are currently
in use by a Delivery.
Removed extraneous entry in CSV header
Merge remote branch 'origin/production' into production
Minor tweak to rendering CSS for LTI launches

Now sets explicit background-color and color just
in case tool provider open things in a frame with
odd colours.
Merge remote branch 'origin/master'
Merge branch 'production'
Added findTestFeedbacks() methods to test/part
Added front-end JS confirm for ass/del deletion
Added methods for deleting users