[feature/avatars] Pass phpbb_user object to prepare_form_acp()

The phpbb_user object might be used for language variables. Pass it as a
function argument to prepare_form_acp() instead of using globals.

PHPBB3-10018

phpBB/includes/avatar/driver/driver.php

@@ -79,7 +79,7 @@ abstract class phpbb_avatar_driver implements phpbb_avatar_driver_interface
 	/**
 	* @inheritdoc
 	*/
-	public function prepare_form_acp()
+	public function prepare_form_acp($user)
 	{
 		return array();
 	}