[preview]: Quiz and worksheet preview moved to core/response

All roles seem to need to see previews at some time or the other
[solution]: Showing solution in place to students and teachers

If the solution is shown only as a preview, then
  1. there is no need to allow PDF download - great for copyright protection
     and for not being seen as a question bank
  2. Enforcing submission from students. Only students who send scans - even if blank -
     should be able to see solutions to that quiz