Adapt validate.py for new fields

[luisa-beerboom]

Closes #49

Couldn't find documentation about what rules relate to the new fields, so I laboured under the assumption that everything in the models.yml was formatted correctly. I doubt that this is actually the case since I also found some type errors in it which I haven't fixed.

Based on what I saw in the models.yml I made the following assumptions about the new rules, which I am unsure about:

• Relation fields need to have at least one of to, reference and sql
• Other number fields may also have to or sql

I have no idea how the unique attribute that was hinted at is supposed to work as it is no where in the models.yml, so I didn't build in anything related to that for now

@r-peschke can you give some more information?

[r-peschke]

Talked about it and decided to close in favor of https://github.com/OpenSlides/openslides-meta/pull/84