cleanup: use DateTime to format dates

Closes #270

Modifications:
- replace custom date parsing by DateTime calls
- use proper date formatting for RSS feeds

Deletions:
- linkdate2timestamp()
- linkdate2rfc822
- linkdate2iso8601

Signed-off-by: VirtualTam <virtualtam@flibidi.net>

tpl/dailyrss.html

@@ -2,7 +2,7 @@
     <title>{$title} - {function="strftime('%A %e %B %Y', $daydate)"}</title>
     <guid>{$absurl}</guid>
     <link>{$absurl}</link>
-    <pubDate>{$rfc822date}</pubDate>
+    <pubDate>{$rssdate}</pubDate>
     <description><![CDATA[
         {loop="links"}
         	<h3><a href="{$value.url}">{$value.title}</a></h3>
@@ -13,4 +13,4 @@
         	<br><br><hr>
         {/loop}
     ]]></description>
-</item>
+</item>