R1:1e918eecfaab

Use standard subscribers effects in Herald Adapter for tasks

Summary: Ref T8455. Use the standard effect in task rules, instead of a custom effect.

Test Plan: Wrote a Maniphest CC rule, updated a task, saw rule activate properly.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8455

Differential Revision: https://secure.phabricator.com/D13182