Move diff styles to separate stylesheet
Position + and - correctly
Tidy up diff view

* Reduce red and green contrast against black text, it was very hard to
read. Use lighter colours.
* Put + and - in a column down the left side
* Don’t put diff in a well, use an empty bootstrap panel with a
modified border
Merge pull request #229 from alphagov/tidy-diff

Tidy up diff between publication editions