Add box-shadow to nav-pills on snapshot#show page

This helps the navigation pill stand out more when it is rendered on top
of the image. I used the same light-dark trick that we did with the
cluster indicators to make it stand out on both dark and light
backgrounds.
Remove the before and after PNG from snapshot#show page

Now that we have the three part sprite diff image, the need for the
before and after snapshots below the diff was minimal. I've replaced the
images with three links: one to the baseline snapshot, one to the before
image PNG, and one to the after image PNG.
Add "anchor" keyboard targets to diff

I've added a top and a bottom "anchor" keyboard-focusable element to the
diff. The top one will have a down-arrow and the bottom one will have an
up-arrow. This helps when you have a diff with only one change for
instance.