hgweb: move HTTP content types out of header templates This removes the ability for templates to add custom HTTP headers, which can easily be re-added if someone needs it. Thanks to asak for repeatedly reviewing this patch and helping to iron out the quirks.

<?xml version="1.0" encoding="{encoding}"?>
<rss version="2.0">
  <channel>
    <link>{urlbase}{url}</link>
    <language>en-us</language>