Problem export should adhere to problem archive format

[eldering]

As reported by Josse van Dobben de Bruyn, an exported problem zip doesn't adhere to the problem archive format we use for importing problems. In particular, the testdata is not in the correct subdirectory.

Further feature requests:

• support export of jury submissions
• export and reimport should be idempotent, e.g. by making sure we write out testcase.desc files.

[eldering]

Multi-file jury submissions in subdirectories are imported since ea54ce00e30beda7e34ef83f3fea7570b506c0f1.

[meisterT]

I think this is fixed. I imported a problem, exported it, imported the export and exported it again and compared the two exports. The diff looks reasonable to me (there's a timestamp in problem.yaml and the submission folders have changed name since the submission ID is part of the name).

[meisterT]

There are still some issues:

• [x] output validators are missing
• [x] filenames are not properly prefixed with 0s, so order would change

Also we decided to add an integration test for this.

[meisterT]

@vmcj suggested to allow also exporting the team submissions in the same structure as we use for importing submissions.

[vmcj]

.interaction files in samples are not exported.

[nickygerritsen]

.interaction files in samples are not exported.

Nor should they I think? We don’t even import them or need them

[meisterT]

It should be complete now, but we should do some more testing, in particular add something to the integration tests.