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
Merge pull request #335 from alphagov/clearer-diffs

Improve legibility of diff views