Ported/updated candidate session mgmt MVC
Improved handling of terminated sessions

There's now a CandidateSessionTerminatedException, which is
intercepted by MVC controllers and forces a redirect to the
assessment renderer, which display an appropriate message.
Fixed some old "CandidateItemSession" naming
Cand summary MVC now shows LTI results only
Added LTI reporting status to MVC (LTI only)
Minor documentation improvement
LTI result reporting now joined up with new code
Tidy-up of LTI result handling
Tidied up candidate session MVC
Backported new candidate session stuff to old MVC.
Imprvoved handling of unavailable launches
Removed temp LTI result code
Added friendlier JSP for candidate launch bailouts
Updated LTI dashboard JSP for new functionality
Added "delete candidate sessions" functionality