Improve display of branched guide

[durden]

Currently it's not very obvious when a reader is viewing a branch of a guide. The URL has ?branch= in it but that's the only way to really tell. There are a number of ways we could improve this:

1. Show some kind of banner (maybe just a simple flash message) denoting that we're viewing a branch, not the original.
2. Do the diff (or get it from github API) and display the branched guide text with embedded notes where a change was suggested

Number 2 is much more difficult to implement, but it's a much better UI. I'm not exactly sure of the implementation details, but the end result could look something like the following. Assume we have a guide with the following text:

This is an original guide. Please read it.

Then, assume there's a branch called xyz that changes the guide text to this:

This is an original guide. I would appreciate you reading it.

The branched view could look something like this:

This is an original guide. I would appreciate you reading it. [1]

Then, you could hover over the [1] and see a link to the diff on github or maybe show the original sentence on hover. I'm sure there are better/cleaner ways to show this information. However, this would be a big improvement on what we have now.