Change usernames to sort64 to make them case-insensitive

Summary: Ref T1191. After adjustment, usernames currently end up case-sensitive, which means `alincoln` and `Alincoln` are different users. Make them case-sensitive so these names collie.

Test Plan: Ran `bin/storage adjust`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10786

src/applications/people/storage/PhabricatorUser.php

@@ -114,7 +114,7 @@ final class PhabricatorUser
     return array(
       self::CONFIG_AUX_PHID => true,
       self::CONFIG_COLUMN_SCHEMA => array(
-        'userName' => 'text64',
+        'userName' => 'sort64',
         'realName' => 'text128',
         'sex' => 'text4?',
         'translation' => 'text64?',