feat(preview): add command to cleanup preview folder

feat(preview): add command to cleanup preview folder

When changing preview parameters (size, format, …), one can simply delete the preview folder and run
files:scan-app-data in order to retrigger updated preview recreation because files are absent.

However, that's not possible when using ObjectStorage as primary storage, as there's no concept of
"preview folder".

This command allows to simply delete and recreate the preview folder properly from
Nextcloud, so that the abstraction levels handle all the subfolder & file deleting part.

Signed-off-by: Thomas Citharel <tcit@tcit.fr>