[WIP] Resurrect the removed visual diff support

This does what it used to do.
Probably brings back the same concerns which caused it's removal in the past
 - whatever those were.

( @tilgovi ? )

What is missing is the stying of the removed and added parts. (It used to be pasten green and ping bg, but you can get creative on that.

Feedback welcome.

We should use the `$sanitize` service on the `diffHTML` before trusting it. While it is our code that generates this diff, it's also our code running in someone else's frame and a carefully crafted postMessage is all it would take to inject arbitrary HTML into our sidebar with this.