Adjust tests for more confidence before refactor

Some tweaks to make the tests around change history clearer and more
thorough before I extract the code to a separate object.
Replace change_history with DocumentHistory

For confidence that behaviour is maintained, we just swap out the
implementation for now.
Make DocumentHistory enumerable
Tidy up use of change history
Cut out redundant tests

DocumentHistory is now unit tested separately so these tests are
redundant.
Extract helper for comparing changes
Include supporting page changes in history
Exclude already-covered supporting page chganges

Those supporting pages that were migrated will share a public
timestamp with their parent policy, which should already have an
appropriate change note. Therefore, we exclude any supporting change
changes from the history that share a public timestamp with a change
in the main policy document.

This also adjusts the wording of the change history generated for the
first published supporting page.
Merge pull request #1119 from alphagov/61314162-change-notes-for-supporting-pages

Ready for merging now - 61314162 change notes for supporting pages

https://www.pivotaltracker.com/story/show/61314162