61314162 change notes for supporting pages

**Note: Not to be merged until the cleanup task in https://github.com/alphagov/whitehall/pull/1115 has actually been run, otherwise the change history will include lots of crurd.**

This reworks the change history code so that changes on a policy's supporting pages are also included in its history.

The first half dozen commits extract the code for the existing behaviour to a new `DocumentHistory` class to make mixing in supporting page changes easier. The final two commits add the new behaviour.


Tracker: https://www.pivotaltracker.com/story/show/61314162