Update experiment to be 50/50

Because the experiment now has different cohorts we need to reset the
experiment name - the alternative would be attempting to migrate all the
old cohorts over to the new bucketing automatically, which was deemed
to be too much development work given the user need.
Refactor code and introduce JSLint

- Removes duplication of the same method
  traversal (`window.location...`)
- Introduces the correct front matter to get JSLint to understand our
  code
- Changes to appease JSLint
Add new copy paths

As part of this I've simplified the code paths.
Merge pull request #605 from alphagov/ukvi-50-50-cohort-test

Change UKVI cohort test copy and frequency