Summary:
Depends on D19012. Ref T13053. In D19012, I've changed "Thread-Topic" to always use PHIDs.

This change drops the selective on-object storage we have to track the original, human-readable title for objects.

Even if we end up backing out the "Thread-Topic" change, we'd be better off storing this in a table in the Mail app which just has `<objectPHID, first subject we used when sending mail for that object>`, since then we get the right behavior without needing every object to have this separate field.

Test Plan: Grepped for `original`, `originalName`, `originalTitle`, etc.

Reviewers: amckinley

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D19013