Allow setting of "required":false in CustomFields

Summary: Fixes T6265, allows you to pass required:false as a parameter.

Test Plan: Add required:false to a field, no longer see "Required"

Reviewers: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6265

Differential Revision: https://secure.phabricator.com/D10659