Cf. comment on PLANET-6625

“Take a note that Page View event fires on the gtm.js event which fires before the consent event. That's why the first collect? hit doesn't have gcs a parameter and  gid is the same on any page. Let's try to fire event with consent before gtm.js - it should help”

Can we send the ‘consent’ arguments before the GTM script is loaded? That should fix our last issue. 

Fire consent mode before gtm.start

GTM starts in templates/blocks/google_tag_manager.twig, included at the top of html_header.twig

Consent mode is fired later, included in the bottom of the page, and requires the window.p4bk_vars also declared at the bottom of the page.

This ticket probably requires us to declare window.p4bk_vars at the top of the page.
We could also declare gtag functions at the top and use them in cookies.js, so that we don't duplicate functions.