Skipped state on submissions

Basically this replicates https://github.com/kursitet/edx-ora/commit/e977927917b7f5a968ba40c680ceb6a6a30f0d24
Which should be enough to make the 'skip' button work as expected. I think.
Disables ML more or less forever.
Empty answers get replaced with a message indicating the answer was actually empty.
Translating an error message.

Why isn't this i18n in the first place anyway.
Now that there are 'skipped' submissions, filter needs to account for them.