Thumbnails Cleanup Maintenance

## Is your feature request related to a problem? Please describe.
https://owncloud.dev/services/thumbnails/#deleting-thumbnails

```
As of now, there is no automated thumbnail deletion. This is especially true when a source file gets deleted or moved. This situation will be solved at a later stage. For the time being, if you run short on physical thumbnails space, you have to manually delete the thumbnail store to free space. Thumbnails will then be recreated on request.
```

## Describe the solution you'd like
Have an intelligent cleanup command like https://owncloud.dev/services/storage-users/#cli-commands

## Describe alternatives you've considered
Just deleting all thumbnails from time to time.

## Additional context
We'll be probably implementing a `rm -rf /var/lib/ocis/thumbnails` cronjob in the oCIS Helm Chart https://github.com/owncloud/ocis-charts/issues/227