Use application PHIDs for application transactions

Summary: Ref T2715. Ref T3578. Load application transactions through application PHID infrastructure.

Test Plan: Viewed feed, saw successful loads of application transaction objects and rendered feed stories.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2715, T3578

Differential Revision: https://secure.phabricator.com/D6617