View in/out/ok on submission details page if user can view tests

[AlexVasiluta]

If the user can view/download tests, show the (currently problem editor-only) column for downloading subtest output, while also adding the test input and output for easy navigation.

One caveat is that the subtest is visible for only a limited period of time. If the subtest is not available, it might be better to transform the link into normal text, maybe with a glossary explanation (like stdin/stdout on the problem statement page), explaining why the output is not available anymore.

[AlexVasiluta]

Can be considered an extension of #43