newProblem gets fired on Exercises, not Khan.

Fixes debug mode.

Test plan:
locally, Chrome/OS X
- Visited http://localhost:8000/exercises/addition_1.html?debug and
  http://localhost:8000/exercises/angles_of_a_polygon.html?debug and clicked on
  problem history, random problem, and different problem types.

Auditors: eater, alpert