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