Skip API call to /attempt if we are previewing an exercise card.

Test Plan:
- (With changes in tom-new branch)
- Load a tutorial in editing mode
- Click on an exercise to preview it
- See that entering answers doesn't trigger an API call

Reviewers: alpert

Differential Revision: http://phabricator.khanacademy.org/D2634
Skip API call to /hint if we are previewing an exercise card.