Rewrite djblets_images tags to use storage backends.

When I wrote these utilities (cropping and thumbnailing), I didn't take into
account that Django provides a storage abstraction. The default storage is on
the local filesystem, but there are third-party backends that also work. This
change updates them to use the storage APIs instead of working directly with
file objects on the local disk.

This change is in preparation for a corresponding review-board change to allow
the use of django-storages backends (specifically Amazon S3) for screenshot
storage.

Tested screenshot thumbnailing against screenshots stored in Amazon S3