Add ManualChangeHistory model
Fix error message for manual guidance/ prefix
Use String#start_with? to implement starts_with?

It makes more sense to use a standard library method than to reinvent
the wheel.
Add manual-change-history artefact and validation

A manual-change-history artefact will contain change history for a
manual artefact. The app which publishes the manual artefact will create
and update the manual-change-history artefact as necessary.
Merge pull request #175 from alphagov/manual-change-notes

Manual change notes