Move 'set' config option type to new structure

Summary: Ref T12845. This move 'set' options (a set of values).

Test Plan: Set, deleted and mangled 'set' options from CLI and web UI.

Reviewers: chad, amckinley

Reviewed By: amckinley

Maniphest Tasks: T12845

Differential Revision: https://secure.phabricator.com/D18160