text: Title: Feeds RSS/Atom
  Body: Implement RSS feeds
  
  https://github.com/pythonhub/quokka/blob/master/quokka/core/views.py#L228
  
  ContentFeed should return a specific Content SubContents (useful for contentblocks and to return images in a gallery)
  
  ChannelFeed should return all the contents inside that channel, if homepage it return all the Contents.
  