See diff with previous editions

This allows users to view the changes between consecutive editions:

![screen shot 2014-08-29 at 12 55 43](https://cloud.githubusercontent.com/assets/3687/4089824/734835ae-2f73-11e4-8837-c5beb0d0a1f9.png)


It also updates the "history & notes" tab to list the version history consistently with the way publisher does:

![screen shot 2014-08-29 at 12 36 40](https://cloud.githubusercontent.com/assets/3687/4089678/dab3c0bc-2f70-11e4-96f0-396bbb96a40c.png)

Due to markup fixes, the pull request is easier to review with whitespace changes ignored: https://github.com/alphagov/travel-advice-publisher/pull/71/files?w=1

Story: https://www.agileplannerapp.com/boards/173808/cards/5695
This is only diffing the summary of these editions.  They also have zero or more parts (very few have zero).  We'll need to include this content in the diff as well.  In publisher, for example, guides are flattened before being diffed.  This is achieved by concatenating all the parts, and inserting the part titles as H1's before each part (eg https://publisher.preview.alphagov.co.uk/editions/53da5c4f40f0b635eb000053/diff).