MDL-33136 Dndupload filemanager - user asked to rename/overwrite file when droping file with existing name

Note this has been applied both to individual activities grade_grades
and to general ones.

This reverts commit 25ccaf54.

Revert "MDL-31863 Web Service : get_files() optional $modified timestamp returns files changed after given time."

This reverts commit 130d3fbf.

And remove a stray #

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.)