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/Key.php

@@ -0,0 +1,13 @@
+<?php
+
+class Key {
+
+    /** @var string The $_REQUEST key for teh current user's e-mail address */
+    public const string USER_EMAIL = 'FRC_USER_EMAIL';
+
+    /** @var string The $_REQUEST key for the current user's ID */
+    public const string USER_ID = 'FRC_USER_ID';
+
+    /** @var string The $_REQUEST key for the array of user messages to display */
+    public const string USER_MSG = 'FRC_USER_MSG';
+}