Role prerequisites

Require a volunteer to meet certain prerequisites (e.g., certification, membership, has volunteered x times already) before being allowed to fill a role.

One possible way to model this is to associate a role with a smart group, though we may need to build some SQL-building forms to support some cases.

An alternative may be to create an alternate interface to define contact subtypes and fields. Each contact subtypes represents a role. Each field represents a prerequisites (e.g., has degree in X, has certification in Y). Could use cron to update the fields.