Sending ManualChangeHistory to Panopticon

- added a manual change notes artefact formatter
- find mappings for Panopticon by slug
- add the formatter to the wiring
- add a test for sending the change notes to Panopticon
Remove PublicationLog for Manual

When we view the updates page, we don't care about the changes to the
individual Manual, only the Manual's documents.
Merge pull request #123 from alphagov/send-manual-change-notes-to-panopticon

Sending ManualChangeHistory to Panopticon