Add survey markup and `hasSurvey` method to the homepage view

- renders the survey in the place of the breadcrumb if we're on the homepage (and thus not showing the breadcrumb)
- Adds a controller test that's really just there to capture the fact that we want our homepage view to include the survey
Style the survey link

- Adding in `.content-centered` to reuse the replicated centering css we use on content and breadcrumbs and etc. Perhaps this should be a mixin?
Merge pull request #722 from alphagov/our_survey_says

Our survey says