Style the RSS feed with an XSLT

[onli]

This adds the pretty-feed-v3.xsl from https://github.com/genmon/aboutfeeds to the RSS page. It does so via the rss.php, as something in our code breaks the xslt line when it is added to the .tpl directly, the ?> gets removed.

As a result, instead of raw xml code the user sees this when clicking on the rss link:

Do not merge this before https://github.com/genmon/aboutfeeds/issues/34 is solved, the license is incompatible. I assume this is an accident, if not we would have to write a new .xslt.