This commit implements the necessary core hooks to ensure we only
allow a regrade of a quetion attempt to take place if the new and old
versions of the question are sufficiently similar.
It will be followed by commits to each question type where the
new method needs to be implemented.
Automated tests will be included in the first of those (mulitple choice)
becuse we need a question type that implements the hooks to test
the core changes.
Tim Hunt
ca4ac3e8b6