Add basic DSB Premiumanzeige workflow including BPMN-driven process data entity model definition.

This includes basic implementation of generating process data entities from BPMN forms.

Validation is disabled completely because it would trigger too early right now. We're having discussions regarding soft validations, content hints etc.

There are lots of missing features (e.g. advanced data types like people, files, richtext, etc.; i18n etc) for which we'll work on in separate tickets.

For #318 (closed).