ExternalBackendForm: make the variable a webserver assigns a username to configurable

refs #12164

application/forms/Config/UserBackend/ExternalBackendForm.php

@@ -55,6 +55,18 @@ class ExternalBackendForm extends Form
                 'validators'    => array($callbackValidator)
             )
         );
+        $this->addElement(
+            'text',
+            'username_envvar',
+            array(
+                'label'         => $this->translate('Username Environment Variable'),
+                'description'   => $this->translate(
+                    'The environment variable the webserver assigns the authenticated user\'s name to.'
+                ),
+                'required'      => true,
+                'value'         => 'REMOTE_USER'
+            )
+        );
         $this->addElement(
             'hidden',
             'backend',