Respect "can edit username" in registration UI

Summary:
Fixes T3741. The flag is respected in terms of actually creating the account, but the UI is a bit unclear.

This can never occur naturally, but installs can register an event which locks it.

Test Plan:
Artificially locked it, verified I got more reasonable UI;

{F81282}

Reviewers: btrahan, datr

Reviewed By: datr

CC: aran

Maniphest Tasks: T3741

Differential Revision: https://secure.phabricator.com/D7577

src/applications/auth/controller/PhabricatorAuthRegisterController.php

@@ -326,13 +326,20 @@ final class PhabricatorAuthRegisterController
     }
 
 
-    $form
+    if ($can_edit_username) {
-      ->appendChild(
+      $form->appendChild(
         id(new AphrontFormTextControl())
           ->setLabel(pht('Phabricator Username'))
           ->setName('username')
           ->setValue($value_username)
           ->setError($e_username));
+    } else {
+      $form->appendChild(
+        id(new AphrontFormMarkupControl())
+          ->setLabel(pht('Phabricator Username'))
+          ->setValue($value_username)
+          ->setError($e_username));
+    }
 
     if ($must_set_password) {
       $form->appendChild(