Move diff calculating process to happen on snapshot return rather than on compare call #8
Description
Currently the image diff process is kicked off by selecting the two versions of the site to compare in the comparison web interface.
This should be moved so when a snapshot is returned, a quick audit looks for other versions of the site and automatically creates the diff in the background, leaving all diffs ready to go very soon after the end of snapshotting rather than that being the beginning of the generation process.