Round 3 of aggregator improvements:

 - Added support for new tags:
   + Optinal feed image: <image> tag.
   + Dublin core dates: <dc:date> <dcterms:created>, <dcterms:issued>,
     <dcterms:modified>.
 - Usability improvements:
   + On the administration page, made the feed/bundle titles link
     to the feeds/bundles' pages.  On the feed/bundle's page, made
     the 'Last updated' field link to the administration page.
   + Moved the 'syndication' menu one level down.
 - Updated some content sensitive help.
 - Further improved themeability.
 - Fixed some invalid HTML.