Atom feed validation fix

I am rewriting the Atom XML feed for the site so that it validates properly. I think this was causing an issue with the feed pulling new posts into Feed Wrangler (and likely others).

Process:

1. Before merging the pull request, test out this branch on the *live* site (push to S3). It works on the development version, but it is important to check that it pulls in feeds to RSS readers as desired.
2. If there is anything wrong with it, push the master branch to S3 immediately and wait for a fix.
3. Repeat if necessary, and once it works, merge and push master to S3.