Add only what is a feed or links one; refuse the rest (#102)

Adding an address looked for the feed a web page links and, finding none, added the address
as it was: every check then failed, and the sidebar called it a feed that had moved. cnn.com
is one; its page links no feed. find_feed replaces feed_behind_page: the address is added if
it is a feed or an OPML list, the feed its page links if it is a web page that links one (and
that is a feed), and otherwise the add is refused with the reason, from the web page (400, the
dialog stays open) and from `ipx add`.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ui/fixtures/nofeed.html

@@ -0,0 +1,2 @@
+<!doctype html>
+<html><head><title>No Feed Here</title></head><body>A site that links no feed.</body></html>