Fix for issue #37

I've added Spring's CharacterEncodingFilter to set the encoding
of incoming data to UTF-8 if it hasn't been supplied by the browser,
which fixes this. Also set accept-charset="UTF-8" on the candidate
forms too for consistency.
Fixed MVC routing target when deleting deliveries

This was still pointing to the old "list deliveries" action,
which was absorbed into the "show assessment" action. Now fixed.
Delivery deletion now removes LTI link candidates

Recall that LTI link candidates are created especially for a delivery.
We now delete all of these users when a delivery is deleted, which is
the correct thing to do with regards to privacy and data protection.
(The original behavious was to keep the candidates in the DB, which
was an oversight.)