i18nize a bunch of javascript files

Summary: i18nize (most of) interface.js, history.js, khan-exercise.js, word-problems.js

Test Plan: Visit the site, see that it doesn't break

Reviewers: john, csilvers, eater, alpert

Reviewed By: john

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D2362