We originally wrote the feature to ask donors for an additional recurring donation as a variant, and it can still be triggered by adding a variant parameter to the querystring.

However, it can now also be enabled per country via a server-side setting.

Let's remove it from the variant framework now that fr-non-tech seems to have settled on version _011 as the best. We can keep it accessible via QS by adding a new monthlyConvert boolean variable to check for.

Further, we have special code in gateway-specific JS files outside of the monthlyConvert javascript which refers to it when handling the return from the donation api call. We should instead call a hook which the monthlyConvert script can attach to.