Change order of link to before and after image

These were in the wrong natural order. In all other places, the before
image is ordered before the after image.
Move diff UI to its own partial

The main snapshots/show haml file was getting hard to read. By moving
the details of the diff UI to its own partial, we make it easier to add
stuff to the main haml file.
Always show the snapshot image on the snapshot#show page

A recent commit (8b94b558094) removed the before and after image from
the snapshot#show page. This was a win in case the snapshot was in a
state where it had a non-zero diff because we saved a lot of space by
not rendering the before and after images. However, when the snapshot
had a zero diff and when it didn't have a baseline to compare against,
you wouldn't see the snapshot image at all.

I added the image back in two places. I also added an alert to explain
the state when there is no baseline to compare against.

The spec coverage for all of this is not great. But I did add some smoke
tests just to make sure that the page would render with snapshots in
different states. And while doing that I found a bug in the snapshot
factory that I fixed.