Refactor dataset updates but dont support private datasets
This PR contains the refactoring needed for supporting the private datasets (https://github.com/huggingface/datasets-server/pull/2224), saving the support for private datasets itself.

The most important part is that we now control a dataset only with:
- operations.update()
- operations.delete()
It's where we check if a dataset is supported (blocked, private, viewer is disabled, etc)

Once a job has been created, we consider the dataset as supported, and we don't check it anymore in the next steps.

Also:
- refactor the tests a bit to speed up the e2e tests, and to reduce complexity
- remove two unused (I think) admin endpoints: /backfill-plan and /backfill (and remove them from admin UI)
- delete the assets and cached_assets from a dataset when we call operations.delete_dataset, which removes the need to run a cron job to delete obsolete assets
- the backfill job is different: it only checks the current entries in the database to complete them if needed, or delete them if they are not supported anymore. It does not allow anymore to add a dataset that was not in the database. Anyway, if a dataset page is loaded, and the dataset is missing from the cache, a new job will be created immediately, so... not so important, I think.
- more tests
Refactor dataset updates but dont support private datasets: This PR contains the refactoring needed for supporting the private datasets (https://github.com/huggingface/datasets-server/pull/2224), saving the support for private datasets itself.

The most important part is that we now control a dataset only with:
- operations.update()
- operations.delete()
It's where we check if a dataset is supported (blocked, private, viewer is disabled, etc)

Once a job has been created, we consider the dataset as supported, and we don't check it anymore in the next steps.

Also:
- refactor the tests a bit to speed up the e2e tests, and to reduce complexity
- remove two unused (I think) admin endpoints: /backfill-plan and /backfill (and remove them from admin UI)
- delete the assets and cached_assets from a dataset when we call operations.delete_dataset, which removes the need to run a cron job to delete obsolete assets
- the backfill job is different: it only checks the current entries in the database to complete them if needed, or delete them if they are not supported anymore. It does not allow anymore to add a dataset that was not in the database. Anyway, if a dataset page is loaded, and the dataset is missing from the cache, a new job will be created immediately, so... not so important, I think.
- more tests