Hide "X created Y, a subtask of P." feed stories again

Summary: Fixes T12787. Modular Transactions don't actually support shouldHideForFeed(). I'll add some discussion to the task.

Test Plan: Created a subtask, saw no more "X reopened Y, a subtask of P" feed story.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12787

Differential Revision: https://secure.phabricator.com/D18058