Configurable duration (in days) for AB Test cookie.

[ikennaokpala]

Related to this PR alphagov/smart-answers#2655

Here, I have added a new option (cookieDuration), this option help to set the cookie expiration duration in days.

When this option isn't supplied the the default cookie duration is set to expire in 30 days.

Motivation

The motivation here is, by reducing the cookie expiration will allow returning users to be assigned
to different cohorts during A/B tests.

According to the performance analyst, A/B tests performance is measured using sessions and with
a default 30 day expiration, they will only see the one variant for that period. Being able to
reduce this increases the likelihood of users seeing different variants across their sessions.

Sample code

  new GOVUK.MultivariateTest({
      el: '.get-started a',
      name: 'startButton_201607',
      customDimensionIndex: <customDimensionIndex>,
      cookieDuration: 14, // set to 14 days
      contentExperimentId: <contentExperimentId>,
      cohorts: {
        original: { callback: function() {}, variantId: 0},
        next: { html: 'Next', variantId: 1 },
        continue: { html: 'Continue', variantId: 2 }
      }
    });

Sample outcome

[dsingleton]

Thanks @ikennaokpala, being able to set a duration for the experiment is a really useful feature, and was needed recently in some other work. So 👍 to the feature idea.

Could you add a test for the new behaviour? There's some prior art for checking cookie values, so I think it should be easy.

One small, non-blocking, point: Is days is the best unit for the duration? Recently we needed a duration in minutes, rather than days, which was possible with Maths, but makes me wonder if we should use seconds, would that be more consistent?

If there's no consensus on that, i'd be more than happy to merge with days, but worth discussing until theres tests.

[dsingleton]

👍 i'm fine with the duration being in days, it's a sensible default and if someone needs to set a lower duration then the caller can calculate a fraction value.