Fixes 4742: snapshot and bulk delete endpoints and task

[dominikvagner]

Summary

• Two endpoints for manual snapshot deletion added, one for deleting a single snapshot and a bulk delete one.
• Endpoints only soft-delete snapshots and then start a task cleaning those up fully along with Pulp content and updating templates that are using those.

Testing steps

1. Create a repository, with snapshots enabled.
2. Add the repository to a template and make sure it uses the snapshots that's gonna be deleted.
3. After the repository has a snapshot created, change the URL and trigger a new snapshot.
4. Delete a snapshot from that repo with the new endpoint.
   http DELETE :8000/api/content-sources/v1.0/repositories/${REPO_UUID}/snapshots/${SNAP_UUID}
5. Verify that the snapshot has been deleted (i.e. not present when listing snapshots for a repo).
6. Verify that the template snapshot has been updated to the other one. (Doesn't work yet.)

• For bulk delete, do the same with more snapshots.

[jlsherrill]

https://issues.redhat.com/browse/HMS-4742

[rverdile]

I'm noticing an issue you can reproduce like this:

1. Create a repository, change the URL to get 2 snapshots.
2. Delete the more recent snapshot.
3. In the UI, if you try to view the snapshots you'll see "No snapshots yet". I believe because in the API, if you fetch the repository, last_snapshot_uuid is omitted from the response.