Add cookie policy with gtag

[samhotep]

Done

Demo for new cookie policy that supports google consent mode. DON'T MERGE.

QA

• Check that before selecting cookie preferences, the section of the site has the default gtag consent values (all denied)
• Check that after selecting cookie preferences i.e functionality or performance, the gtag values are updated correctly in the section.
• They should correctly match the essential, functionality or performance selections, as specified here

[webteam-app]

Demo starting at https://canonical-com-1225.demos.haus

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.14%. Comparing base (49c790a) to head (a4b0718). Report is 17 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1225 +/- ## ========================================== - Coverage 71.45% 66.14% -5.31% ========================================== Files 12 10 -2 Lines 1023 898 -125 ========================================== - Hits 731 594 -137 - Misses 292 304 +12 ``` | [Flag](https://app.codecov.io/gh/canonical/canonical.com/pull/1225/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | Coverage Δ | | |---|---|---| | [python](https://app.codecov.io/gh/canonical/canonical.com/pull/1225/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical) | `66.14% <ø> (-5.31%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=canonical#carryforward-flags-in-the-pull-request-comment) to find out more.

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