Serving images to external sites uses renditions

Previously, an image served to an external site was rendered on the fly.
This may have performance issues with large websites.

This commit changes the image serve view to use renditions so images are
only rendered when the url is first accessed. When the user hits the
url, they are now given a permanent redirect to the renditions image.
Added preview view to admin

As the frontend serve view now uses renditions, previews are now
generated with their own view to prevent loads of unneeded renditions
being created.
Send the file from the view without redirecting
Front-end cache is no longer necessary for image generator
Release note for #753
Merge branch 'kaedroho-image-frontendserve-refactor'