Remove google analytics tracking from iframes used in Drupal course about pages

This fixes two issues:
 - if the course pointed to doesn't exist, GA was added to the iframe twice
 - more importantly, we don't want to count these as separate views anyway--
   we just care about the about page itself.
 - I also added a note about where we would put our segment.io widget if we want it
   later

LMS-2581
Merge pull request #3551 from edx/victor/remove-iframe-ga

Remove google analytics tracking from iframes used in Drupal course abou...