Fix duplicated date when importing news via RSS.

#6079 

This PR adds a view mode for "news" that is used in the "OS news" view in feed display mode.

Now when importing a news item form one OS site to another we get only the publication date:
![selection_202](https://cloud.githubusercontent.com/assets/4497748/3076479/6bad7a2e-e3e3-11e3-93ad-3ac3022d4a1a.png)

And in the path ``edison/news.rss`` we don't have the date in the feed <description> tag:
![selection_203](https://cloud.githubusercontent.com/assets/4497748/3076480/95710ea2-e3e3-11e3-9dd8-e7885e588e02.png)