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

scripts/search/reindex_maniphest.php

@@ -26,6 +26,7 @@ $tasks = id(new ManiphestTask())->loadAll();
 echo "Updating relationships for ".count($tasks)." tasks";
 foreach ($tasks as $task) {
   ManiphestTaskProject::updateTaskProjects($task);
+  ManiphestTaskSubscriber::updateTaskSubscribers($task);
   echo '.';
 }
 echo "\nDone.\n";