Move analytics values into variables.

[antfitch]

I've moved the analytics values into variables in site.yml. This now matches what we do on the Flutter side of things.

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[dart-github-bot]

Visit the preview URL for this PR (updated for commit 5ef18c9):

https://dart-dev--pr6654-google-analytics-add-variables-q9fzotlf.web.app

[parlough]

Looks good to me! Thanks for consolidating these.

Note that to access the global site. variables, I had to change the render calls for these partials to include.

[antfitch]

Looks good to me! Thanks for consolidating these.

Note that to access the global site. variables, I had to change the render calls for these partials to include.

Thank you so much for doing this!