Add a default "Subscribed" query to Maniphest

Summary: Although I don't want to end up with 20 of these again, this is a reasonable default to provide, particularly for installs where a large portion of the userbase primarily reports bugs and does not interact with them directly.

Test Plan: Hit /maniphest/, saw "Subscribed", clicked it, saw the tasks I'm subscribed to.

Reviewers: jbrown, btrahan

Reviewed By: jbrown

CC: aran

Maniphest Tasks: T4100

Differential Revision: https://secure.phabricator.com/D7586