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