New endpoint to massively create elements and images

The goal is to have an endpoint capable of creating a large list of elements and images in a single call, replacing lots and lots of calls to CreateImage and CreateElement.

The endpoint would be named CreateIIIFElements and would only support:

• internal users
• iiif urls
• providing width & height (no checks on the backend)
• images are considered validated (that's why only internal users are allowed, we can't safely open that to workers for now)
• no worker version id
• support polygon
• only one parent

The payload would have the following fields:

• parent_id
• list of elements + images descriptions:
  • iiif_url: full url towards the image
  • width and height of the image
  • polygon (nullable, when null, use full image, according to width & height)
  • name of the element
  • type of the element (should support ID and slug here, using a pre-flight check/load)
  • metadatas as a dict of key => value

The endpoint would create in a transaction:

• the new images, using a bulk create
• the new elements (a pre-fligh check is needed here to list only the new elements)
  • if an element with this parent + polygon + image exists, skip the rest
• the metadatas on new elements (keep it simple here, only adding metadatas to elements that were created)
• the element paths on new elements (should not be hard, as you can load initially all the required paths on the parent)

This is needed to speed up a lot socface and other massive ingestion processes (like callico 🤞 )