Allow late submissions after deadline

Currently, after the submission deadline has passed for a project, IVLE will forbid any further submissions.

This is not useful in practice, as there are always a number of students who submit late for various reasons, and not all will request a formal extension (and we don't necessarily want to have to grant extensions either -- extensions are for students who have permission to submit late without penalty).

IVLE should allow submissions for a project even after the deadline has passed (so the deadline has no physical impact on the system, only on the UI). The UI should change to reflect this:

* The lecturer view should clearly show late submissions with an asterisk, and give the number of days late.
* When the submission has closed, the submit UI should read "The deadline has passed. You may still submit late, but a penalty may be incurred."
* On the actual POST request to make a submission, if the submission will be late, it should NOT be made immediately. The confirmation screen should be a prompt which says again "The deadline has passed. You may still submit late, but a penalty may be incurred." and a button marked "Submit Late", which sends another POST request with an extra flag "late=yes". This prevents race conditions where a student may believe they are submitting on time (within seconds) but actually submits late (maybe they have already submitted and feel the new material is not worth the mark penalty). This mirrors physically having to submit to "proj.late" in the old CSSE submit system, to confirm your intent to submit late.