Allow diffing solutions in AWS

[prandla]

This is something I want to implement soon. I'm not sure about how to make a good UI for selecting the two submissions to diff though. It ought to be possible to select two submissions from different pages of the paginated submissions list, I'm not sure how to implement that...

I'm going to say this is blocked on #1351 because once we have prism.js, we can get really nice syntax highlighting on the diffs. (I have the code for 1351 written already, just need to make a PR...)

[veluca93]

My 2c: at least to begin with, it's probably enough to diff with the previous solution of the same contestant.

For more complicated scenarios, maybe it's fine to just ask for two submission ids?