Fix external js updates to multi-selects

I ran into some issues where updated multi selects from external
javascript would frequently get out of state because the .choices
variable on the Chosen object was no longer the same as the actual
number of options selected.

Instead of having a global variable on the chosen jquery object
(which is relatively fragile because of external changes to the
select element), just calculate it as needed. It's not called often
and this is a fairly fast lookup for jquery and prototype, so I think
this is a safe change.