Allow backspace to be caught in CS scratchpads

Summary:
When a user presses backspace, send keyCode 8 to keyPressed
instead of navigating to the previous page in the browser's
history.

This adds an e.preventDefault() call on a backspace keypress
on the scratchpad.

Test Plan:
 * Modified https://en.khanacademy.org/math/applied-math/cryptography/crypt/p/polyalphabetic-exploration
 * Saved as http://jack.local:8080/cs/backspace/5691896819089408
 * typed things and pressed backspace to delete them

Reviewers: john, pamela

Reviewed By: john

Differential Revision: http://phabricator.khanacademy.org/D6308