website/www: use absolute links in the feed

Some feed readers, like FeedBurner[1] don't support content
that uses relative links. This just makes them absolute so they
will work everywhere.

[1]: https://support.google.com/feedburner/answer/79018?hl=en
Merge pull request #2996 from pearkes/website-rss-absolute

website/www: use absolute links in the feed