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
Make it so that ambiguous plural errors can be programmatically re-enabled. Then re-enable them in the lint tests.
Fix the html here -- remove an extra </span>.

This lets HTMLParser parse all of exercises/.  Huzzah!

Auditors: alpert, john

Test Plan:
Ran my htmlparser based string extractor and had it succeed.  But
mostly, manual examination of the html.
Merge branch 'master' of https://github.com/Khan/khan-exercises into absolute-inequalities