(khan-exercises) Replace top-level var declaration with explicit globalization with window

Summary:
A new lint is being introduced as part of the migration to using CommonJS
modules.

See: {D4710}

Test Plan:
`arc patch D4710` within the webapp directory
`arc patch` this revision within the khan-exercises subrepo within webapp directory, then run:

```
tools/runlint.py -l js_css_packages khan-exercises
```

No lint errors should be reported.

Reviewers: alex, alpert

Reviewed By: alpert

CC: alpert, john

Differential Revision: http://phabricator.khanacademy.org/D4730