Update edition comparison view

This improves the diff view and brings it into line with that of travel advice publisher - it now shows a more detailed diff view that is calculated on the fly and highlights individual word changes between lines:

Before
![before](https://cloud.githubusercontent.com/assets/3687/4374277/04461fc6-4333-11e4-90bd-81b128ecb131.png)

After
![after](https://cloud.githubusercontent.com/assets/3687/4374280/088cce72-4333-11e4-8aea-148181ed6f91.png)

Story: https://www.agileplannerapp.com/boards/173808/cards/5970

this `margin-bottom` is causing a lot of unnecessary whitespace between lines. i'm doing a like for like comparison on preview:
http://publisher.dev.gov.uk/editions/53d79f9540f0b642ef000015/diff
https://publisher.preview.alphagov.co.uk/editions/53d79f9540f0b642ef000015/diff