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
Personally, I would say no: I don't think it adds that much additional value in terms of confidence that the app is doing what it' supposed to. If by some chance someone should happen to delete the code below that renders the diffs for the parts, then editors will let us know pretty sharpish that they've gone and the cost of that is very low.