Convert 'class' config options to new validation
Summary: Ref T12845. These options prompt the user to select from among concrete subclasses of some base class.
Test Plan: Set, deleted and mangled these values from the web UI and CLI.
Reviewers: chad, amckinley
Reviewed By: amckinley
Maniphest Tasks: T12845
Differential Revision: https://secure.phabricator.com/D18159