changed exam authentication to use a single key for all students + other changes

- changed exam backup interval from 5 sec to 30 sec
- fixed bug in examEditService authorization check (check that no
students are writing the exam being edited)
- changed color of release date input to white (to reflect the fact
that it's editable, even tho it is set to read-only, because it is
being edited through the jquery datetime picker)
- moved auth key from exam stats page to exam edit page, auth key is
uneditable on exam edit page even if no students have written the exam,
exam must be unreleased to change the key