Test that group are correctly handled by WP form endpoint
Test that WP patch respects group assignment setting
Test that assignable responsibles can contain groups
Test that it is possible to assign groups as responsibles
Test that it is allowed to assign a group as responsible

There isn't a validation for this yet. But also assignee misses
such a validation. I left a TODO, so others are reminded to fix
this globally someday.
Allow to assign groups as responsibles
Validate group assignment is activated