After editing an image, previously cached version is still served

[hc-nolan]

Will soon be rewriting all image handling code and using UUID-based filenames for all images, so I'm not going to spend time trying to implement some cache busting method.

[hc-nolan]

3e24055 - one two line change feels too good to be true but I think this should work fine. New images will use UUIDs and existing ones will still be served fine. Just need to verify that the caching behaviour is fixed