Migrate zones on element

I do not really see a way to split that work 😿

The goal is simple: remove Zone table, and migrate all polygons on the Element table, thus removing unique constraints.

There should be no modification on the API, so nothing will change for our clients (frontend, workers, whatever).

The work would roughly look like:

• create a migration that moves all polygons and remove the Zone table
• fix serializers and API endpoints that reference a polygon
• iterate on all unit tests (they will be badly broken...)
• test on a full dockerized local setup, with some workers
• test on arkindex-dev-web
• ship on preprod

You could split in multiple merge requests toward a branch no-zone that we would merge after some time: CI tests could be allowed to fail on that branch.