R1:aa7ba4c6e6d6

Implement Differential subscribers as a CustomField

Summary: Ref T3886. Now that a custom field can emit a core transaction, just emit a subscribers transaction.

Test Plan: {F116014}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886

Differential Revision: https://secure.phabricator.com/D8289