add two helper account access levels:

- read only - can't subscribe to more feeds, feed updates are skipped
 - disabled - can't login
define used access levels as UserHelper constants and refactor code to
use them instead of hardcoded numbers

classes/handler/administrative.php

@@ -2,7 +2,7 @@
 class Handler_Administrative extends Handler_Protected {
    function before($method) {
       if (parent::before($method)) {
-         if (($_SESSION["access_level"] ?? 0) >= 10) {
+         if (($_SESSION["access_level"] ?? 0) >= UserHelper::ACCESS_LEVEL_ADMIN) {
             return true;
          }
       }