Betty-cropper

- Add storage caching layer (ex: memcached in front of S3 backend) to improve performance with slower storage backends.
Cache time controlled by new `BETTY_CACHE_STORAGE_SEC` setting (default: 1 hour).

- Fix `image.js` function `computeAspectRatio` to always return a valid ratio. Previously there was an edge case where it would
return `undefined`, resulting in a bad image URL.

- Added management command `make_disk_storage_paths_absolute` to convert legacy relative disk paths to absolute paths.

- Fix: Cache flush callback now also includes CLIENT_ONLY_WIDTHS

- Added "If-Modified-Since" / 304 request header support to avoid accessing backend storage on cache re-validation.

**Requires migration (add column `Image.last_modified`)**

- Allow optional alternate cache duration for non-breakpoint crop widths (i.e. width not in `settings.BETTY_WIDTHS` and
`settings.BETTY_CLIENT_ONLY_WIDTHS`) via new `settings.BETTY_CACHE_CROP_NON_BREAKPOINT_SEC`.
- This allows breakpoint-widths to set very long cache times because they will be included in the itemized cache flush callback.
- If `settings.BETTY_CACHE_CROP_NON_BREAKPOINT_SEC` not set, will use `settings.BETTY_CACHE_CROP_SEC`.