Add the user satisfaction survey

This adds the user satisfaction survey banner to the wrapper template.

The survey banner will be displayed randomly to approximately 1 in 6 users. Once the user has clicked "take survey" or "not now", the cookie `govuk_takenUserSatisfactionSurvey` will be stored to remember that they have already dismissed the survey.

When a user clicks the "take survey" button, they're taken to the survey start page (hosted on SurveyMonkey) with the current path on gov.uk appended as the `c` parameter so that we can capture it in the survey response data.

Please don't merge this until the associated changes to the cookie support pages in alphagov/frontend#308 have been merged.