Render Maniphest custom fields last

Summary: Ref T2217. Fixes T3865. Eventually we'll probably make more of this configurable, but for now shove custom fields down instead of sort of arbitrarily putting them in the middle.

Test Plan: Looked at a task with attached revisions and custom fields, saw custom fields last.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217, T3865

Differential Revision: https://secure.phabricator.com/D7103