Allow "edits" to Exam Overrides

[firasm]

Just wanted to pointed out once an override is created, it cannot be edited (for example the dates/times, etc...)

Also, if a course has 10 overrides, you use one, make a mistake and (since you can't edit it) then delete it and create another one, the override counter decrements by 2 (in total). This may be moot if overrides are editable.