jquery-ui isn't available on the site, so make it an exercise util

Summary:
I spent a bunch of time trying to get backbone modals to work, but
wasn't able to get to something as pleasant to use as the jquery-ui periodic
table. Perhaps I have much to learn about backbone, but the case where the
user's browser window was smaller than the periodic table was giving me all
sorts of problems. I wanna ship this today, so meh.

Test Plan:
Tested the chemistry exercises that use jquery-ui via appengine
dev server after adding utils/jquery-ui.js to javascript-packages.json

Reviewers: alpert

Reviewed By: alpert

CC: stephanie

Differential Revision: http://phabricator.khanacademy.org/D1024