Add / Edit Activity: create new contact option for With contact (target) field

Track work for this issue in Bluebird: http://dev.nysenate.gov/issues/5007 (work should be tracked for December 2012).
----------------------
Add 'create new contact' dropdown to the right of the 'With' contact field so users can add new contact(s) inline. Selecting a contact type results in pop-up profile form. Saving the profile form inserts the new contact into the With contact widget.

Users should be able to insert 1 or more newly created contacts into the With field list, AND should be able to combine selecting 1 or more existing contacts with creating new contacts.