Add `export-contest` script

[vmcj]

We already have a script to import from a contest package, it would be good to also have a script for the other way around.

This would help with EOC and make it easier for users to share the contest package. It would also be useful for internal testing to make sure that importing and exporting do work.

I'm not sure if we would need to export DOMjudge only files or that we can actually use a file which is maintained within the CLICS repos.

[eldering]

I was actually working on this in Astana.

[vmcj]

See also #2768 when implementing this.