R1:e3b5737d02fb

Support CustomField in Herald, mostly

Summary: Ref T655. Ref T418. This mostly supports CustomFields in Herald, for conditions only.

Test Plan: {F137845}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T418, T655

Differential Revision: https://secure.phabricator.com/D8695