This adheres more closely to the specs, especially Atom, where the
'description' arg becomes <summary>.
Note that this means full article content will no longer appear in RSS
feeds.
publication time and date and the last modified time and date
independently.
This makes it possible to access the last updated date with {{ article.locale_modified }} in templates.
Additionally, an already delivered feed entry can be corrected by changing the modified date and time, as it is used for atom:update
/ rss pubDate field now.
