Add support for <content:encoded> (RSS)

   See https://www.rssboard.org/rss-profile#namespace-elements-content

   This namespace is recommended by best practices, and can be commonly found
   in the wild.  The content of the <content:encoded>, if available, is
   going to be used to populate EccioEntry::content. The old <description>
   element is used as a fallback.

Complete missing fields

   A few fields of EccioEntry were not properly populated in Atom.  This
   problem has been fixed.