Improve phabot handling of private messages

Summary: When private messaged, the bot responds via private message to the
sender, instead of sending a private message to itself.

Test Plan: Mentioned tasks in public channels and private messages.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T274

Differential Revision: https://secure.phabricator.com/D1350