Fancy dropdowns in perseus

Summary:
Recreates the fancy jQuery mobile dropdowns in Perseus.
Also adds a ~~terrifying~~ awesome <FancySelect> component, which is
everything you ever wanted from a custom <Select> and more. (Uh, does
that mean things you *didn't* want?)

{F28164}

{F28167}

Test Plan:
 * Updated test.html to a live item using the new fancy dropdowns.
 * The fancy dropdowns' text sizing is a little large right now, because
   mobile is the only intended client so far, so they look a bit too big,
   but I also subbed in the mobile css and tried it, and it looks nice
   (the screenshot is from there).
 * Tried grading, both empty and non-empty.
 * Tried putting really long things in the dropdown, and forcibly made
   sure that the dropdown always resized itself accordingly, such that
   when you choose a new option, it doesn't reflow the page.
 * Made sure it still looked pretty after doing the above, which was not
   pretty.
 * #yolo!

Reviewers: joel, charlie, elizabethlin

Reviewed By: charlie, elizabethlin

Differential Revision: http://phabricator.khanacademy.org/D10780