Add "Crop image previews" setting to files

Added a new user setting that toggles cropping on image previews
in grid view.

True (default value): crops each image to a square. False: keep original
aspect ratio.

Signed-off-by: Nina Pypchenko <22447785+nina-py@users.noreply.github.com>

Closes #18439.

Signed-off-by: npmbuildbot-nextcloud[bot] <npmbuildbot-nextcloud[bot]@users.noreply.github.com>