Remove mixeval edit view, using the add view now.
DB constraints, cascading deletes for mixeval.

Deleting a question should now automatically delete the associated
question descriptors. Likewise, deleting a mixeval should also delete
the associated questions.
New style mixeval edit now half working.

Half working because while save works, question/question desc deletion
doesn't work yet.

This mostly just removes the old code while trying to see how much of
the mixeval add code we can reuse.

Turns out that mixeval add can be turned into mixeval edit with a simple
change where hidden id fields are added if it's available. So mixeval
edit now renders the add view.

Deletion looks like it'll require comparing before and after data for
missing questions/question desc. This looks to be a bit more complicated
since the deletions has to be fitted into the transactional save, so I'm
putting it off for another commit.
Mixeval edit now works for question deletion.

Question deletion is done by comparing the data currently in the
database and what the form submitted. If the form submitted data is
missing IDs, then the question or question desc that the ID refers to is
deleted. This is still part of the save transaction, so it will be
rolled back if an error was encountered.