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.
Replace guts of SnapshotComparer#to_array_of_arrays

The first implementation of this iterated over each pixel, building up
arrays of arrays. This commit simplifies this a bit by using ChunkyPNG's
built-in `#row` method, which returns an array of the given row's pixels
as Integers, which is exactly what we want. Since we are using OilyPNG,
this may improve the performance, but even if it doesn't, it is simpler
and easier to read.

At the same time, I added some YARD in the vicinity.

Change-Id: I8c3b4279d7d675681987ca02fb868ab29334c9fd