When sorting subscriber references for display in the curtain UI, sort without case sensitivity

Summary: Ref T13486. Currently, "Zarbo" sorts above "alice", but this isn't expected for a list of (mostly) human usernames.

Test Plan: Loaded a task with subscribers with mixed-case usernames.

Maniphest Tasks: T13486

Differential Revision: https://secure.phabricator.com/D20969

src/applications/subscriptions/engineextension/PhabricatorSubscriptionsCurtainExtension.php

@@ -123,7 +123,9 @@ final class PhabricatorSubscriptionsCurtainExtension
           $handle = $handles[$subscriber_phid];
           if ($handle->isComplete()) {
             $is_complete = true;
+
             $sort_name = $handle->getLinkName();
+            $sort_name = phutil_utf8_strtolower($sort_name);
           }
         }
       }