Add better permissions

The permissions on Quiz Reports are from the original bundled Quiz statistics module, and should be updated. We now have:

access user stats
access author stats

(they don't make much sense, and 'access author stats' is probably not used at all).

I propose we add these permissions instead:

'view any quiz report' = user can see reports for all quizzes
'view own quiz reports' = user can see reports for the quizzes the user is the author of