Add Assignment.submittable attribute to configure student submission.

Add Assignment.submittable attribute to configure student submission.

If `Assignment.submittable` is `True`, mailpipe will accept student
submissions for that assignment.  However, there may also be
assignments where you don't want to accept direct submissions
(e.g. attendance and written, in-class exams).  For these assignments,
you should leave the `submittable` option at its default `False`
value.

Note that `mailpipe` doesn't use this value yet (implementation coming
soon).