Block creating any new elements outside of their parents in the API

Bastien Abadie
2022-04-21T12:37:38Z

Refs https://gitlab.com/arkindex/backend/-/issues/168

This one is header than #976

As suggested on #168; I would prefer to take a low-level approach, checking in `add_parent` that all new element links are inside the parent polygons

Refs https://gitlab.com/arkindex/backend/-/issues/168

This one is header than #976 (closed)

As suggested on #168 (closed); I would prefer to take a low-level approach, checking in add_parent that all new element links are inside the parent polygons