Change RSS content type & use absolute URL in meta

This PR is the result of me trying to stop Google Chrome from displaying this message on our RSS feeds:

![](http://puu.sh/d4NUa.png)

The solution was to change the content type from `text/xml` to `application/rss+xml`. Now chrome displays a plain document. It would be nice to be able to provide some styling and have a pretty RSS feed in the future - that's a note for @halfdan and #3993.

Additionally, whilst looking around at examples of other people's RSS feeds trying to determine what caused Chrome to display the message, I noticed it was far more common to use an absolute URL for the `<link rel="alternate">` tag, so I've changed that too.


----

no issue

- changes the content type for the RSS feeds from text/xml to
  application/rss+xml
- changes the link rel=alternate tag to use an absolute URL for the feed
  in the blog meta data