Add image sizes to question and hints

Summary:
Adds an images object prop to the item, containing a map from image
URL to {width: height:} description of the image

Test Plan:
 * add images to the question and answer area; check their width
   and height attributes in the element inspector
 * add two images to the question area
 * add two of the same image to the question area

Reviewers: alex

Reviewed By: alex

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7946