Added subscriber view to Maniphest.

Summary:
People want to see all the tasks they have subscribed to in one
view. A new table was added for this to make queries faster.

Test Plan:
Tested that the view was initially empty. After running the
reindex_maniphest.php script, I saw the correct tasks there. Added
myself as a subscriber to one task and made sure the view was
updated. Removed myself as a subscriber from one task and made sure
the view was updated again.

Reviewed By: epriestley
Reviewers: epriestley, jungejason, codeblock
CC: aran, rm, epriestley
Differential Revision: 603