Better handling of previews that are out of sync with the file

[artonge]

1. Upload an office file
2. Request a preview
3. Disable previews for office files: occ config:app:set richdocuments preview_generation --type boolean --lazy --value false
4. Modify the file

If the preview is served, it is out of date.

Best would be to disable serving previews for providers that are disabled, or to have a way to cleanup previews for those providers.