introduce constant for configuration directory

This replaces {data_dir}/settings throughout the code, so that
the configuration may be placed somewhere else in the directory
structure (e.g. in /etc).

lib-php/dotenv_loader.php

@@ -5,7 +5,7 @@ require('Symfony/Component/Dotenv/autoload.php');
 function loadDotEnv()
 {
     $dotenv = new \Symfony\Component\Dotenv\Dotenv();
-    $dotenv->load(CONST_DataDir.'/settings/env.defaults');
+    $dotenv->load(CONST_ConfigDir.'/env.defaults');
 
     if (file_exists('.env')) {
         $dotenv->load('.env');