When non-administrators click "Change Username", explain why they can't continue

Summary: Ref T13420. This workflow currently dead-ends for non-administrators. Instead, provide explanatory text.

Test Plan:
  - Clicked "Change Username" as an administrator, same workflow as always.
  - Clicked "Change Username" as a non-administrator, got explanatory text.

Maniphest Tasks: T13420

Differential Revision: https://secure.phabricator.com/D20827

src/applications/people/controller/PhabricatorPeopleRenameController.php

@@ -3,8 +3,13 @@
 final class PhabricatorPeopleRenameController
   extends PhabricatorPeopleController {
 
+  public function shouldRequireAdmin() {
+    return false;
+  }
+
   public function handleRequest(AphrontRequest $request) {
     $viewer = $this->getViewer();
+
     $id = $request->getURIData('id');
 
     $user = id(new PhabricatorPeopleQuery())
@@ -17,6 +22,16 @@ final class PhabricatorPeopleRenameController
 
     $done_uri = $this->getApplicationURI("manage/{$id}/");
 
+    if (!$viewer->getIsAdmin()) {
+      return $this->newDialog()
+        ->setTitle(pht('Change Username'))
+        ->appendParagraph(
+          pht(
+            'You can not change usernames because you are not an '.
+            'administrator. Only administrators can change usernames.'))
+        ->addCancelButton($done_uri, pht('Okay'));
+    }
+
     $validation_exception = null;
     $username = $user->getUsername();
     if ($request->isFormOrHisecPost()) {