feature: assignment cfg: MaxSubmissionSize

We want to limit, by default, the number of bytes that are unpacked from
the archive. One could upload an archive that unpacks to a huge file,
causing a DoS on the checker.

Reported-by: Alyssa Milburn <amilburn@zall.org>
Signed-off-by: Lucian Cojocar <cojocar@gmail.com>