CreateElementChildren endpoint

Closes #1816 (closed)

• The worker_run_id field is not included, since ElementPaths are not linked to worker runs.
• Using the same URL as ListElementChildren and DestroyElementChildren implies using the same APIView with two different serializers, which is really hard to handle as different serializer contexts are needed, different ACL checks are needed, nothing in the OpenAPI schema generation works, etc.
• /api/v1/element/<id>/children/bulk/ is already taken by CreateElements. I went with /api/v1/element/parent/<id>/, which is similar to /api/v1/element/<id>/parent/<id>/ of CreateElementParent, since this is a bulk version of CreateElementParent.

changed milestone to %Arkindex 1.6.2

requested review from @babadie

assigned to @erouchet

added 3 commits

• 8b0974ef...9efef8da - 2 commits from branch master
• 3695a7bc - CreateElementChildren endpoint

Compare with previous version

resolved all threads

A bit slower than I hoped for, but it should be helpful still.

Works as expected through API client.

merged