Highlight paragraphs with widgets

Summary:
This is a first step towards the improved answering experience.
Paragraphs with widgets that have yet to be interacted with are
highlighted.

{F14443}

Test Plan:
* Set the SHOULD_INDICATE_WIDGETS constant to true
* Open up some questions in perseus
* Paragraphs containing widgets you have not interacted with yet should
  be highlighted
* This should also work for widgets in the answer area, as well as
  answer areas that are not "Custom format"

Reviewers: alex, jack

Reviewed By: jack

Subscribers: marcos, alpert, eater

Differential Revision: http://phabricator.khanacademy.org/D7411