Add manual change notes

We want to expose change notes for manuals. A manual lives at `/guidance/:manual_slug` and the change notes for it will live at `/guidance/:manual_slug/updates`.

This pull request adds a new artefact type, `manual-change-notes`, and adds a custom slug validator for it.

Part of https://trello.com/c/mjsAameQ/126-expose-the-change-note-data-from-the-content-api-3