Update draft_submit option to be present in mod_assign form

This option was only being inserted into the mod_assignment form, as we
were checking for the presence of a field called "var4" which does not
exist in mod_assign.  This has been switched to submissiondrafts, which is
also now used to toggle the disabled state of the draft_submit field.
Submit files for assessable_submitted events too

The mod_assign module triggers an assessable_submitted event, which we
were listening for but not processing correctly.  It should be handled the
same way as file_done, as both result in no pathnamehashes being provided,
but there are some differences between how mod_assign and mod_assignment
handle these events.
Make draft_submit option available for mod_assignment too

Commit ab1b285 made this option work for mod_assign, but broke it for
mod_assignment.  This commit makes it work for both.