Create prohibit_during_test decorator

[SimonAdamek]

Teachers sometimes allow students to upload materials for tests to Kelvin. So, I enabled viewing of assignments with a "Hard deadline" that expires before the start of the test.

This means that if a teacher wants to allow students to upload test materials to Kelvin, they must create a task with a "hard deadline" before the test. It's a bit complicated, but I couldn't think of any other way to solve it.

Related to #603

[Kobzol]

Thank you, the decorator looks reasonable. I'm not familiar with the use-case for uploading "test materials" though, I'll ask geordi about it.

[SimonAdamek]

What I mean is, for example, we now sometimes have the opportunity to look at the source codes of older assignments or upload cheat sheets for tests to Kelvin. I don't know how many teachers do this, but I decided to allow it....

I think that if it's there, nothing will happen, and anyone who wants to can use it.

[Kobzol]

I think you already talked about this with geordi, but there are some "edge cases" that we should consider, for example someone uploading a submit e.g. a week before a test, and then reading its code, or someone submitting code through a comment on a different task, and then deleting that comment to hide tracks.

We were talking about making it impossible to delete and modify comments for students during a test. However, that is not enough, because someone could make a comment during a test, then copy the output from it, and then delete the comment just after the test ends (but before someone realistically has a chance to take a look at their submit to see the incriminating comment).

Maybe what we could do is to make any comments that are created ~hour before/after a test be "pinned", so that they cannot be modified nor removed by the student, ever? @geordi

[SimonAdamek]

Currently, I do not allow students to make any comments during the test, or view old ones. I do not understand how it would be beneficial if it were +/- one hour. However, this can be easily adjusted if you want...

[Kobzol]

The problem with "during test" is that quite often tests last longer than the deadline :)

[Kobzol]

Teachers sometimes allow students to upload materials for tests to Kelvin.

If Kelvin is used as a side channel to upload materials for tests where students don't have internet access, we should change that. That is super brittle and I don't trust that we can implement the logic in Kelvin to support that use-case robustly.

CC @geordi what do you think about this.