Remove misleading comment in snapshot_comparer.rb
Simplify SnapshotComparer#compare!

By breaking out some things into a separate method and removing an
unnecessary comment, this method is now shorter and simpler than before.
Make `SnapshotComparisonImage`s use nested module/class

This was pointed out by Rubocop, and I for one am not the type of person
who will stand up against law enforcement.
Use pills instead of tabs for diff image types

I ran into a couple of issues when using the bootstrap `.nav-tabs`
component:

- The tabs were not as visible as I would have wanted
- Setting the width and a bg-color helped, but didn't look good all the
  time.

I'm switching to using the `.nav-pills` component instead. It's more
noticeable and works better without a set width.

I also changed the initialization to happen in js instead of through the
data-spy attribute. It turns out that bootstrap will only initialize
components once, on document ready. Since we're using Turbolinks I had
to make sure that the affix behavior worked even after a turbolink page
change. This had the added benefit of making the transition from static
to fixed positioning smoother.