Resolve relative href during discovery

[sybereal]

Currently, if a page contains a feed link with, e.g., href="/rss.xml" or href="rss.xml", feed discovery will return the relative URL as is, instead of resolving it to an absolute URL.

I don't really have any experience with PHP, but if you could give some pointers on how to implement it. There was an existing PR in nextcloud/news#1631, but that seems to depend on pear/net_url2, and I can understand if you do not want to add another dependency just for this.