Allow a defined static value to be added to, if auto_form_fields has an optional list for the item.

Summary: We use auto_form_fields to suppress the field from being displayed in a form.

Test Plan: None

Reviewers: Web Administration Panel Developers, machniak

Reviewed By: Web Administration Panel Developers, machniak

Maniphest Tasks: T525

Differential Revision: https://git.kolab.org/D27