Fixes https://github.com/feedjira/feedjira/issues/212

When the Atom parser is deciding which of the <link> elements present holds the URL for the feed, look for a <link> with rel="self". When deciding which is the website URL, if no <link> has type="text/html", return the href value for the last <link> in the XML which is not the feed <link>, if possible.
When Atom parser looks for the feed URL, it looks for link elements with rel="self".

I forgot this change in commit cd60f450d100f3637df535464131f14b4932f765.
Simplified url method in Atom parser

It's functionally equivalent but much cleaner. Added sample feed and
test case for the bug being fixed.