Prevent volunteer self-assignment through imported fields

For the established (legacy) Volunteer Management portal. These steps differ from the newer Volunteer Management interface. Administrator controls require the appropriate school or volunteer-management permissions.

Legacy VMS uses Selectable: false prevents volunteer self-assignment. New person-data imports use Unselectable by Volunteer, where True, T, Y, Yes or 1 mean blocked. The fields have opposite polarity; confirm the importer and test a small set. Retain the constituents in the upload. This does not necessarily hide all data or block administrator assignment.