Ensure that `moment.js` always exposes `moment` as a global as it is used by the `khan-exercises` framework.
Ensure that `moment.js` always exposes `moment` as a global as it is used by the `khan-exercises` framework.

Test Plan:
With the other changes to `webapp` I loaded up an exercise and was able to successfully complete a streak of problems.

Auditors: csilvers

Khan/moment,Khan/moment