Modularize Maniphest Herald "Assign Task" action
Summary: Ref T8726. No surprises here.
Test Plan:
- Created rules using this action.
- Applied migration.
- Verified rules still work.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: joshuaspence, epriestley
Maniphest Tasks: T8726
Differential Revision: https://secure.phabricator.com/D13706