Add editing of feed URL (#4)

- Move feed-specific database calls to Feed class
- Detect when feed items have been updated
- Add const keys for $_REQUEST values

src/lib/Security.php

@@ -31,11 +31,11 @@ class Security {
                 die('Unrecognized security model (' . SECURITY_MODEL . ')');
         }
         if (!$user && $redirectIfAnonymous) {
-            header('/logon?returnTo=' . $_SERVER["REQUEST_URI"], true, HTTP_REDIRECT_TEMP);
+            header('/logon?returnTo=' . $_SERVER['REQUEST_URI'], true, HTTP_REDIRECT_TEMP);
             die();
         }
-        $_REQUEST['FRC_USER_ID']    = $user['id'];
-        $_REQUEST['FRC_USER_EMAIL'] = $user['email'];
+        $_REQUEST[Key::USER_ID]    = $user['id'];
+        $_REQUEST[Key::USER_EMAIL] = $user['email'];
     }
 
     /**