Add side-by-side comparisons

[jyn514]

Bolding only goes so far - it doesn't show removed items, modified items, and it's hard to implement. I'd like to do a side-by-side comparison. diff and patch together should make this fairly simple on the backend; on the front end we'll need to add a second panel.

[jyn514]

Found a tool called colordiff which makes this look similar to git diff

only works for terminals, not HTML
In combination with aha, which converts ANSI to HTML, this should work.

[jyn514]

@hdamron17 do we want to show the difference from the last commit, or from the last commit seen in the browser? The first is easy to implement, the second requires brennan to pass things to the backend.

This would destroy our caching, ignore this