Add a "Header" standard custom field

Summary: Ref T418. This is the last of the Maniphest field types, although I have a few more capabilities/options to port over.

Test Plan: See screenshots.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T418

Differential Revision: https://secure.phabricator.com/D7018