First cut of log on page (#9)

- Add session support
- Refactor security handling to use db connection
- Fix db path issue

src/public/user/log-off.php

@@ -0,0 +1,10 @@
+<?php
+/**
+ * User Log Off Page
+ */
+
+include '../../start.php';
+
+if (array_key_exists(Key::USER_ID, $_SESSION)) session_destroy();
+
+frc_redirect('/');