A/B test responsive theme

Redirects proved to be problematic for a) speed and b) form posts, so this code uses the Content Experiment API as described in https://developers.google.com/analytics/solutions/experiments-client-side to swap the styleseets to the responsive ones and rewrite the header and footer as the document is written if the responsive arm of the content experiment is selected. 

Closes https://github.com/mysociety/alaveteli/issues/1244