Use webpack to bundle JS together, not requirejs

Summary: We're now using webpack to build perseus (D11292) and I've never liked this r.js tool. Use webpack here too since it understands AMD wrappers out of the box.

Test Plan: Loaded http://exercises.ka.local/exercises/addition_1.html, built in webapp and loaded addition_1 there too.

Reviewers: jlfwong, jack

Reviewed By: jack

Subscribers: eater

Differential Revision: http://phabricator.khanacademy.org/D11294