Give the unread convention roles a reader, or take them out of the list

Problem/Motivation

The conventions entity declares ten roles a site can fill in, and two of them - special_pages and reviews_rating - are read by nothing. What actually decides those two is configuration under myrest.settings: the special pages service reads special_page_nodes, and the reviews aggregate reads reviews_rating. So one fact has two declared homes and only one of them is wired, and a site that fills the convention in - on a page that offers it - changes nothing at all. That is worse than an absent feature: it is a feature that appears to work.

Proposed resolution

• Decide where each of the two belongs. The model is the direction of this project, so reading them from the conventions and treating the settings key as the fallback is the expected answer - but it is a decision, not a refactor.
• Whichever way it goes, only one of the two homes stays authoritative, and the other is documented as the fallback or removed.
• A role that stays in the list gets a test that filling it changes a response; a role that leaves the list leaves the conventions page with it.

Remaining tasks

Everything.