Add helper for generating atom feed urls

Now that we are verifying feed urls, we need the generated atom feed
urls to match the exact host for the app, i.e. Whitehall.public_host.
This won't be the case in development unless we explicitly set the
host when generating the url.
Don't generate generic documents feed url anymore

This feed is no longer directly accessible for email signups on the
site any more so is defunct. Confirmed with Neil Williams.