Implement validation rules in backend

[praseodym]

Closes #26.

This implements the validation rules from #22 according to the API described in #25.

Also tested together with #31 and this did not break anything.

[lkleuver]

lgtm!
one caveat. I worry that it might be hard to grasp the validation rules for non-coders.

[praseodym]

one caveat. I worry that it might be hard to grasp the validation rules for non-coders.

I don't think we should have a goal of making code understandable for non-coders. The validation rules come from the controleprotocol which is published by the Kiesraad and very readable. Our code will be tested against these and other policies/laws ('wettelijke toets') by a third party, which should give non-coders enough confidence in our implementation without having to read the code themselves.