Currently, only Documents have a `parent_topic` field to represent doc hierarchy. If that's edited, there's no record of the change. If we follow the example of other fields - eg. title, slug, tags, content, etc - the `parent_topic` shouldn't be edited on Documents directly. Instead, Revisions should have a `parent_topic` field, and saving a new current Revision should apply the change to the Document. That will allow us to track change history of this field and revert changes down the road.

Groundwork for page / section move and undo-move. Also need to revise slugs for moved pages and pages whose parents move