MarkingTool access for tutors allowed
- permission validation for MarkingTool added
- permission validation for tutor filter added
- sets tutor filter initially to uid
checks permission for group page (admins only to disable this section)
upload page is accessible if sheet has started and is not expired
strikethrough the "save changes" link
adds exercisesheet data to upload() result
adds the possibility to generate custom error messages
negative IDs for new exercises
unused file removed (editSheet.php is contained in createSheet.php)