Acceptance criteria:
- Add a new page property "book referencing".
- Add the page property to any page that includes the new attribute, upon save.
- Should be possible to search for all pages which have the property attached.
- With tests.
We're not sure how difficult it will be to remove the page property once the attribute is removed. If it's easy, include in this task. But we're thinking that each parser hook only has access to a small piece of wikitext, so cannot determine whether the attribute was not used elsewhere in the page.