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