Declare summary of atom feed items as HTML

RFC4287 Section 3.1.1 requires HTML content to be declared
as such with the "type" attribute, this commit adds that attribute
to the generated atom feeds.

includes/pages/user_atom.php

@@ -77,6 +77,6 @@ function make_atom_entry_from_news($news_entry)
             page_link_to('news_comments', ['nid' => $news_entry['ID']])
         ) . '</id>
     <updated>' . date('Y-m-d\TH:i:sP', $news_entry['Datum']) . '</updated>
-    <summary>' . htmlspecialchars($news_entry['Text']) . '</summary>
+    <summary type="html">' . htmlspecialchars($news_entry['Text']) . '</summary>
   </entry>' . "\n";
 }