DbUserBackend: Add case insensitive filter column `user'

refs #8826

library/Icinga/Authentication/User/DbUserBackend.php

@@ -32,6 +32,7 @@ class DbUserBackend extends DbRepository implements UserBackendInterface
      */
     protected $queryColumns = array(
         'user' => array(
+            'user'          => 'name COLLATE utf8_general_ci',
             'user_name'     => 'name',
             'is_active'     => 'active',
             'created_at'    => 'UNIX_TIMESTAMP(ctime)',
@@ -39,6 +40,13 @@ class DbUserBackend extends DbRepository implements UserBackendInterface
         )
     );
 
+    /**
+     * The columns which are not permitted to be queried
+     *
+     * @var array
+     */
+    protected $filterColumns = array('user');
+
     /**
      * The default sort rules to be applied on a query
      *