Maniphest - make custom fields always render last

Summary: Fixes T6029. We should append custom fields last so they show up after things like projects, tokens, etc that render via UI events.

Test Plan: viewed a task with custom fields and projects was last

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T6029

Differential Revision: https://secure.phabricator.com/D10407