Rework site cookie banner for compliance about wagtail.org HOT 3 OPEN

👍 added the label! @vossisboss what are your thoughts on this?

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The current site needs an "opt out" button, I'd argue this is a legal requirement as the site is currently loading Google analytics in via Google Tag Manager regardless of consent.

Options to consider:

1. Custom cookie banner
   A custom integration with the additon of an opt out button, similar to the torchbox.com site using js-cookie https://github.com/torchbox/torchbox.com/blob/37f8258b0439ffc13b252833d38b5ccf28734dd8/tbx/static_src/javascript/components/cookie-message.js#L4, I'm not sure how that;s setup to work with GTM but it seems to work fine and is something we could do in an accessible way fairly quickly.

2. No cookie banner (privacy focused analytics)
   Remove the cookie consent banner altogether using "privacy focused analytics", both https://plausible.io/blog/made-in-eu or https://withcabin.com/ look to be good options.
   The paid version also for Cabin also has carbon reporting which would be a nice to have, they also monitor page weight so it's much more accurate then estimating based of page weight averages and page views alone see.
   Depeneding on how and if the current google analytics data is being used, I'd really like to push for going with Cabin straight away, and doing something more custom if needed later on.

3. Third-party tool
   We review and pick a third party tool to manage the cookie banner.

Note: We may also be able to remove GTM (80KB) if it's only being used to load Google analytics. Plausible and Cabin are also a lot lighter in file size then google analytics.

@thibaudcolas It would be great to have an issue tag/label for sustainability in this repo along with in Wagtails, this will help us keep track of sustainability improvements and help keep us more accountable https://docs.wagtail.org/en/stable/advanced_topics/sustainability_considerations.html.

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It's green too!

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