R1:0e6756775c13

Support placeholder text in Herald

Summary: Ref T4420. We don't currently pass placeholder text properly, but should.

Test Plan: Saw placeholder text in Herald.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4420

Differential Revision: https://secure.phabricator.com/D9901