Improve legibility of diff views 

New lines were displaying as thin strips. Spaces between lines made making comparisons more difficult. The +/- signs were hard to read against grey text.

* Give each line in the diff a minimum height, making new lines display thicker and maintaining a vertical rhythm
* Remove space between lines, the content naturally provides this spacing
* Extend red/green highlight to edges of the view
* Use darker red and green colours for the +/- icons

Based on https://github.com/alphagov/publisher/pull/335

## Before
![screen shot 2014-12-23 at 15 02 26](https://cloud.githubusercontent.com/assets/319055/5539142/b82241a4-8ab5-11e4-87c1-60326ef03951.png)

## After
![screen shot 2014-12-23 at 15 02 42](https://cloud.githubusercontent.com/assets/319055/5539143/b838b6aa-8ab5-11e4-9b95-cda75ac58b73.png)