Add analytics dimension for A/B tests

[suzannehamilton]

Link meta tags named govuk:ab-test to a custom Google Analytics dimension. The value of the meta tag and the dimension is a combination of the test name and bucket name, such as EducationNavigation:B.

This links to existing session dimension 13, which has been used for A/B tests in the past. We're not going to use the existing A/B test dimension (13) so that it can continue to be used by JS-based A/B tests. Tara has reserved dimension 40 instead.

Note that only one A/B test can use this dimension at once, otherwise the value will be overridden by different tests.

Trello: https://trello.com/c/g9OyC7Pn/310-tell-google-analytics-about-a-b-tests

• Check with a performance analyst that we can use this dimension, and ideally create a few more so that there's a pre-existing list that teams can use when creating a new A/B test.
• Update the meta tag on the A/B example page in frontend to match the expected structure (Make A/B test meta tag consistent with Google Analytics JS frontend#1110). When I added it, I thought it would just be used by the Chrome extension, but it's simpler to update the tag and the extension than to add more logic to this analytics code.
• Update the Chrome extension to match (Update A/B bucket parsing to match new meta tag format govuk-browser-extension#57).