Convert "enum" and "string" config options to new modular option types

Summary: Ref T12845. This moves the "enum" and "string" types to the new code.

Test Plan: Set, deleted, and tried to set invalid values for various enum and string config values (header color, mail prefixes, etc) from the CLI and web.

Reviewers: chad, amckinley

Reviewed By: amckinley

Maniphest Tasks: T12845

Differential Revision: https://secure.phabricator.com/D18156