Add interface to allow users to search for volunteers

The attached mockups illustrate the enhancements.

The first image displays new icons that will be added to the Assign Volunteers interface. To minimize visual clutter, the icons will be hidden until the user mouses over the shift block. (The mockup shows what the user would see when mousing over the Audio shift block.)

When the user clicks the search icon, a new pop-up appears allowing the user to search for contacts. The search fields will include Groups as well as the custom fields in the Volunteer_Information custom group, which extends the Volunteer contact subtype (see VOL-55). The user can select contacts from a search result set to fill volunteer needs.