Add option to scratchpad CKEditor plugin for delaying scratchpad activation until scrolled to

Test Plan:
Open an article
Edit scratchpad, check this new 'start on scroll' checkbox
Save article
Load preview of article
Using dom inspector, verify that URL contains '&autoPlay=false' and iframe has a data-start-on-scroll=true attribute.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D11304