Feature summary: When VisualEditor/Citoid automatically populates the url field for a {{cite journal}}, also add a url-access=subscription field, unless the DOI can be confirmed to be libre/gratis OA on Unpaywall.

Use case(s): VisualEditor/Citoid populates the url field automatically after expanding DOIs and other identifiers. Such URLs should be added when they're paywalled, according to English Wikipedia documentation: «Online sources linked by |url= [...] are presumed to be free-to-read. When they are not free-to-read, editors should mark those sources with the matching access-indicator parameter». A previous request to fix the issue by avoiding such problematic URLs has been rejected (T232771), so now we need to handle those URLs, but no automated method has been identified. (Due to rate limits and other technical measures, it's impossible to know for sure whether an URL provides a full text or not, or even to resolve most paywalled URLs automatically to convert them to the corresponding DOI to be checked on Unpaywall for the same.)

Benefits: Reduce the GIGO in citations which contain fields whose meaning is ambiguous and which bots can't fix because there's no way of knowing why the URL was added in the first place. Increase access to knowledge. Nudge editors into setting the correct access parameters and OA parameters.