Add a "schedule task" trigger action

Summary: Ref T6881. Add a standard "just queue a task" trigger action; I expect almost all application code to use this.

Test Plan: Will test in Instances.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11429