Remove custom sorl key-value store

As far as I can tell, it was added to circumvent a problem in Sorl. The
problem was fixed in
https://github.com/mariocesar/sorl-thumbnail/pull/167, and we use a
released version that contains the fix.

I also don't see it being used anywhere. So it seems safe to delete it.
Allow disabling deletion of image files and thumbnails

Oscar by default deleted image files and thumbnails. That is usually
fine, but has been an issue when using remote storages like S3. This
change also makes the only remaining import from sorl optional, hence
paving the way to making it completely optional.

Fixes #1373.