MDL-32783 quiz overdue handling: enforce end of grace period
The summary page was not enforcing the end of the grace period. If the
user had the summary page open, then they coudl still stubmit after the
end of the grace period.
Also, the editing form was not validating that the quiz grace period was
greater than quiz|graceperiodmin in the quiz configuration, and that
should be checked, so I added it.
I fear that processattempt.php is becoming very spaghetti-like with all
the timing rules, it really needs to be refactored, but not 2 weeks
before the 2.3 release. (When refactoring, we really need unit tests for
this.)
