Refactor TeX react component into it's own file

Test Plan:
 * Add an expression widget
    * type in an expression, and make sure it renders
 * Add a transformer
    * translate the polygon, and make sure the
      translation renders with latex <, >s
 * Add a multiple choice widget, and make sure that
    * `$x + 3$` renders correctly
 * Make sure that `$32 * 6y$` renders correctly in
   the main question area

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5848