Tidied up LTI exit functionality

(I didn't have a LTI TC to test this in when implementing on Saturday,
so was doing this blind!)
Fixed minor regression in 'try assessment' invoker
Fixes and tweaks to new LTI auth/identity code

The information stored in the LtiAuthenticationTicket is now immutable,
consisting mainly of entity IDs. My first implementation stored the
detached entities, but these of course went stale very quickly!

I've also fixed the 'exit' action to ensure that the ticket is removed
from the session.
Minor tweak to system/anon authn

These now store the ID of the authenticated user, rather than the
resulting (detached) entity. This is simply to bring it in line with how
LTI authn works, there's no change in functionality here.
Created some temp AdhocAction function in eng mgr

This temporary functionality is being used to tidy up unfinished
candidate session after the BEMA test pilot at Edinburgh. Some of this
functionality should make it back into the proctoring interface.
Ready for 1.0-beta5