bug fixes + authentication finish

- finished student's side of authentication in order to write exam
- copying html items with html entities caused errors (&nbsp; etc)
- mc items with img/audio options preview would display them
horizontally even if source is available
- mc items with empty html source, but saved original source, wouldnt
hide the left cell even though it is empty
- when releasing exam, delay is only added if keys are being generated
and saved