Allow users to set preview quality

[SimplyCorbett]

Preview images can use a lot of space and for some reason there is no way to delete generated previews from within nextcloud on the admin end.

To get around this I have previews set to a low quality. The downside is shared images look bad. I don't mind the quality in the gallery since I can just download the direct image.

So my suggestion is:

Allow a global preview quality setting that only applies to shared images.

Alternatively allow users to pick quality per-image and force the preview to regenerate. This should not be automated or possible while uploading the images. This will allow the server to still use 50%-60% quality on most images and allow the nextcloud user to generate higher quality previews when needed.

The second should be possible since as it currently stands previews are never generated again. The first suggestion might require more work.

Cheers!