Prevent pagetree's jQuery from interfering with the main app's copy

Even though pagetree and my django app (worth2) are using the same
version of jQuery, since worth2 is using requirejs it was causing
jquery-ui to not get loaded correctly, breaking the pagetree admin
interface.

This commit loads jQuery in a more cautious way that prevents
interference, in the same way that django-debug-toolbar does it:
    https://github.com/django-debug-toolbar/django-debug-toolbar/blob/69bf1eb127f8c1b6b7c6c2e798ccd208293c1309/debug_toolbar/templates/debug_toolbar/base.html#L7

Also:
- Update jquery-ui to 1.11.2
In worth2 I tried just using the application's jQuery, but since it's loaded with requirejs, I got errors when loading jquery-ui. I don't think it's a good idea to rely on the outer application's jQuery because then we make an assumption that jquery is included as a script tag in the application's `base.html`.. which might not be true.