Use ApplicationSearch in Herald

Summary: Ref T2769. Ref T2625. Herald is currently a giant mishmash of hard-codes and weird special cases. Move toward modernization and normality.

Test Plan: {F52716}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625, T2769

Differential Revision: https://secure.phabricator.com/D6652