Change mechanism for identifying responsive site

Spoke to Ammar about a better way to be able to identify/track responsive pages.  The original suggestion (appending '| Responsive' to the title of the page) would have been noticed by users (sighted and blind).  Ammar suggested adding the following code to each page:

dataLayer.push({
  'Responsive page': 'Yes',
  'event': 'Responsive page'
});

I've updated the page template to add this object to the dataLayer array.

@andrewtennison