Allow custom fields to be reordered and disabled from Config

Summary: Ref T1703. Put a more reasonable UI than "blob of JSON" on top of this.

Test Plan:
Reordered, enabled and disabled user profile fields.

{F49317}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1703

Differential Revision: https://secure.phabricator.com/D6393