[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.

> > [...] the showDiff field is being persisted on the save [...]

> See my comment here on trying to stop that, using the approach suggested by you earlier.

The link got lost in transition. [Here](http://https//github.com/hypothesis/h/pull/1709#issuecomment-64057203).