Add date for RSS feeds (#4)

- First cut of user message add/display

src/lib/Data.php

@@ -118,10 +118,10 @@ class Data {
      *
      * @param string $url The URL for the RSS feed
      * @param string $title The title of the RSS feed
-     * @param string $updatedOn The date/time the RSS feed was last updated (from the XML, not when we checked)
+     * @param ?string $updatedOn The date/time the RSS feed was last updated (from the XML, not when we checked)
      * @return int The ID of the added feed
      */
-    public static function addFeed(string $url, string $title, string $updatedOn): int {
+    public static function addFeed(string $url, string $title, ?string $updatedOn): int {
         $db = self::getConnection();
         $query = $db->prepare(<<<'SQL'
             INSERT INTO feed (