[#48243391] Fold history of multiple editions

Provide a JS accordion to minimise/expand edition comments to make
reading the history of a specific artefact with multiple editions
easier to understand and consume.
Wrap accordion UI elements in the Twitter Bootstrap style

This provides a little arrow showing the UI affordance.

Also using slideToggle(), over slideUp() and slideDown() to reduce
some of the login in our JS files.
Merge remote-tracking branch 'origin/fold-history'
Set id to be #edition-history so it's more specific

Having this set to #accordion before wasn't portable.
Merge pull request #97 from alphagov/auto-save-edition

Moving the previous comments into a new story so merging.
Add beforeSend 'Saving' message