Issue #3623983: Give the unread convention roles a reader, or take them out of the list

The reviews_rating role is read: the aggregate rating is counted over the bundle whose conventions fill the role, with the rating field of the role and the service field of its settings, and the myrest.settings key stays as the fallback for a site that has not filled it. The scanner proposes the role on the bundle of reviews under the names the aggregate reads, and the cached rating now depends on the role and the setting as well as on the reviews. The special_pages role leaves the list: a special page is one node of the whole site in one role, not a property of a bundle, and myrest.settings:special_page_nodes is where the special pages service reads it. The scanner's suggestions for special pages are applied to that key from the queue, and a page somebody already chose is not replaced.

Closes #3623983