Changes Analytics API library to accept one, both or neither of the
analytics tracking codes. This means we can start removing classic
tracking codes from apps.
Fix: Before this fix, when a user fell into variant 0 of a multivariate test,
the data wouldn't be reported to Google correctly because of a broken
null check in the code block that opts the user into the Google Content
Experiment.
Add: Analytics API https://github.com/alphagov/govuk_frontend_toolkit/pull/162 https://github.com/alphagov/govuk_frontend_toolkit/blob/master/docs/analytics.md