Upgrade to 1.0.0-rc856

Concerns: pushword/newsletter

Click tracking, doubly opted in

Campaign and drip links can be rewritten to record which contact clicked which link — only when the audience's new clickTracking switch is on and the contact carries a dated clickTrackingConsentAt; otherwise mails keep their links exactly as before. New table and columns: run the schema update. A site that leaves the switch off has nothing else to do — see the consent model.

Sites overriding confirm.email.html.twig that switch tracking on: the consent is asked in the confirmation mail through a second link, carried by the new confirmTrackingUrl template variable — mirror it or keep a single button.