Conpherence - grey out username mentions if they aren't in the conpherence

Summary: Fixes T7578. This was pretty easy because conpherence funnels all transacton stuff through this spot

Test Plan: made a new room so only my user was a participant. wrote "@myself will work and @anotherguy will be greyed out" and so it was as expected

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7578

Differential Revision: https://secure.phabricator.com/D12114

src/applications/conpherence/view/ConpherenceTransactionView.php

@@ -143,7 +143,8 @@ final class ConpherenceTransactionView extends AphrontView {
     $handles = $conpherence->getHandles();
     $rendered_transactions = array();
     $engine = id(new PhabricatorMarkupEngine())
-      ->setViewer($user);
+      ->setViewer($user)
+      ->setContextObject($conpherence);
     foreach ($transactions as $key => $transaction) {
       if ($transaction->shouldHide()) {
         unset($transactions[$key]);