Change expression to use PropCheckBox

Summary:
Also move the simplification warning under the appropriate checkbox and
adds a prop to PropCheckBox for which side of it the label should go on.

{F12943}

Test Plan:
* Check out test.html
* Check the box and see it show up when you have an unsimplified
  expression!

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6622