Improvements to ballot entry drag/drop support.

* Downloads jQuery UI from Cloudflare (since other resources are from
there)
* Rearranges “reason” textboxes as well as just the votes
* Better visual indications: gmail’s move/grab hand cursors, border for
placeholder box
* Sizes the placeholder row correctly to match the current ballot row
height (should account for fluid layout)
* Sorts out the numbering issues caused by the placeholder element in
some browsers