Implement drf-spectacular for better OpenAPI schemas

Probably should be done after the DRF 3.12 upgrade and overrides rewrite (#478 (closed)) as we might not need it that much. This starts with getting up to date with how the OpenAPI development is going; the drf-spectactular package came from a pull request that was too large and opinionated to be immediately merged into DRF, although some features were included later. The maintainer might be still trying to slowly merge things into DRF; this needs to be checked.

drf-spectactular should, in theory, allow us to complete %Supprimer patch.yml by allowing for all the overrides we might need.