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/public/index.php

@@ -9,8 +9,6 @@ include '../start.php';
 
 Security::verifyUser();
 
-page_head('Welcome');
-?>
-
-<?php
+page_head('Welcome'); ?>
+<p>Unread items go here</p><?php
 page_foot();