Surface edits to "Text" panels on dashboards as remarkup edits

Summary: Fixes T13456. These edits are remarkup edits and should attach files, trigger mentions, and so on.

Test Plan: Created a text panel, dropped a file in. After changes, saw the file attach properly.

Maniphest Tasks: T13456

Differential Revision: https://secure.phabricator.com/D20906

src/applications/dashboard/xaction/panel/PhabricatorDashboardTextPanelTextTransaction.php

@@ -9,4 +9,14 @@ protected function getPropertyKey() {
     return 'text';
   }
 
+  public function newRemarkupChanges() {
+    $changes = array();
+
+    $changes[] = $this->newRemarkupChange()
+      ->setOldValue($this->getOldValue())
+      ->setNewValue($this->getNewValue());
+
+    return $changes;
+  }
+
 }