added exam editing for released exams

- exams that have been released can have their: name, instructions,
alloted time, start window, release date modified, as well as the exam
unreleased IF no one has written or is in the process of writing the
exam
- results can also be released or unreleased from here (make full
results available to students regardless of whether anyone else is
writing the exam, or if the results have been disabled)