Have modular transactions fall back correctly when selecting feed titles

Summary: Ref T9789. Falling back to parent:: is better, and fixes older-style feed stories for Pastes, like "added a comment".

Test Plan: Viewed a comment feed story about a paste.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9789

Differential Revision: https://secure.phabricator.com/D16114