Don't show self-subscribes in feed or mail
Summary: These transactions (when a user subscribes or unsubscribes only themselves) are universally uninteresting.
Test Plan:
- Subscribed/unsubscribed, saw transactions but no feed/mail.
- Commented, got implicitly subscribed, saw only comment in feed/mail, saw both transasctions on task.
Reviewers: chad
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D14853