Resolve TMSS-173 (2nd try)
First of all, sorry for my erroneously labeled commit messages (with SW-907). Please assume any commit on this branch was actually for task TMSS-173
I reviewed Joern's mixin solution (which I suggested in the first place), and considered it way too complicated (again, I know I suggested in the first place). So, I made a far simpler solution, and applied it to all models with json schemas and template.
The validation works, because of course all the existing tests broke, which exactly proves why we want validation. So, I fixed the tests by providing valid json schemas and data.
Finally, I tweaked the tmss-test-setup such that it properly exits (code=1) when database setup fails. I was triggered to do that because of the failing validation because of incorrect json data in the populate scripts. This again proves why we want validation.