Allow submitting exercises late

[jamo]

• Allow submitting exercises late, but by default don't give points for late submissions.
• Allow downloading exercises, whose deadline has passed (tmc-netbeans)

[mpartel]

When determining whether and exercise was late, be sure to compare against the submit time instead of the sandbox completion time.

[rce]

I'll take a look at this one.

[jamo]

Now thinking again it's a bit iffy whether we still need it; since we have the soft and hard deadlines...

[jamo]

I still can see some usecases for this though.