Moved answer area preview next to answer area editor.

Summary:
Made question and hints previews exactly 480px wide.
Outlined answer area preview to match question and hint previews.
Made "Acceptable Formats" and "Check Answer" buttons more realistic.
Added borders to AnswerAreaEditor to match question and hint editors.
Moved AnswerAreaEditor and AnswerAreaRenderer to a separate file.
Removed lots of unused styles, refactored and commented the remainder.

Test Plan: Verified that functionality didn't change via test.html and local dev server.

Reviewers: jack

Reviewed By: jack

CC: alpert, eater

Differential Revision: http://phabricator.khanacademy.org/D4675