Only disable scratchpad when subhints are explicitly included

Summary:
For T2529
In a separate commit, I'll move the intervention header above the scratchpad.
It's not possible to change the z-index of just the links to show subhints
since they appear inline in paragraphs that we want below the scratchpad :(

Test Plan:
1. Include these changes in web app
2. Go to http://stable.ka.local/exercise/divisibility_tests
3. See that the scratchpad is disabled
4. Go to http://stable.ka.local/exercise/addition_1
5. See that the scratchpad is available and works

Reviewers: jack

Reviewed By: jack

Subscribers: alpert, jascha

Differential Revision: http://phabricator.khanacademy.org/D9105