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
See the code below. :)