Flesh out OPDS generation backend to be more robust and to support generic Atom feeds

lib/RssFeed.php

@@ -0,0 +1,20 @@
+<?
+class RssFeed extends Feed{
+	public $Description;
+
+	public function __construct(string $url, string $title, string $path, string $description, array $entries){
+		parent::__construct($url, $title, $path, $entries);
+		$this->Description = $description;
+		$this->Stylesheet = '/rss/style';
+	}
+
+	protected function GetXmlString(): string{
+		if($this->XmlString === null){
+			$feed = Template::RssFeed(['url' => $this->Url, 'description' => $this->Description, 'title' => $this->Title, 'entries' => $this->Entries, 'updatedTimestamp' => (new DateTime())->format('r')]);
+
+			$this->XmlString = $this->CleanXmlString($feed);
+		}
+
+		return $this->XmlString;
+	}
+}