Highlight empty widgets on check answer

Summary:
As per the dogfood, the changes the highlighting feature in a few ways:

* Only highlight once the user has selected check answer
* Highlight specific widgets - not paragraphs

The paragraph stuff is still in there, I'll leave it in for now, in case
we decide we want to do something else with it, but I'll probably remove
it once this is done.

{F15629}

Test Plan:
* Open perseus, add a few widgets
* Click score
* See the ones you didn't enter things into light up!
* Repeat for answer area
* `make all`, repeat

Reviewers: alex, jack

Reviewed By: jack

Subscribers: eater, marcos, alpert

Differential Revision: http://phabricator.khanacademy.org/D7897