Add button to re-compare snapshots

This button will run a new diff against the baseline without fetching a
new snapshot. This will mostly be useful for when we are working on
tweaks to how we are generating the image comparisons, but there may be
some real-life situations where it will come in handy.

Change-Id: Iffec5f7f474bd879baa2df9434d06e50e0b74271
WIP: Use diff-lcs gem for diffing

I'm trying out the diff-lcs gem to manage diffing instead of our own
home-cooked solution. It looks promising, but I still haven't figured
out how to best use it. My first proof-of-concept treats all changes the
same way, and does not draw the semi-transparent original image as the
background.

Some specs are failing, but I'm not too worried about those at the
moment.