TMSS-917: primary subtask

It proved quite difficult to have the "constraint" of only one primary subtask per parent task_blueprint.

In order to achieve that I had to rewrite TMSS-568 such that the parallel calibrator and target LBA subtask are now in one single parent TaskBlueprint, but defined in two subsections in the json schema. That rewrite was much bigger than the "primary" subtask stuff.

Reintroduced the not-null constraint on the parent TaskBlueprint for the Subtask.

Also had to adapt many tests for the new restrictions.

All in all, we now have a much more strict but usable model.

Closes TMSS-917