Cancel existing plus subscription automatically on Pro subscription

Closes minds#2211

Had to hack around the payment system quite a lot to propose a solution - after this runs it does run the "cancel" line, but the subscription still remains. Is there something that I am missing or should this work?

I note another couple of reports yesterday where users seemed to be unable to cancel subscriptions to support_tiers and wondered if it could be related.

Queries I found useful to test

select * from minds.subscriptions;

UPDATE minds.entities SET value = '123123'  where key='1234817396389187587' and column1='pro_expires';

INSERT INTO minds.subscriptions (plan_id, payment_method, entity_guid, user_guid, subscription_id, amount, interval, last_billing, next_billing, status) VALUES ('wire', 'usd', 1224415251189993473, 1234516244091113490,   'urn:subscription:stripe-1234516244091113490-1224415251189993473',  6000, 'yearly', '2021-04-29', '2021-05-06', 'active');