convert importfeed to youtube-dl

Fully working, including --fast/--relaxed.

Note that, while git-annex addurl --relaxed is not going to check
youtube-dl, I kept git annex importfeed --relaxed checking it.
Thinking is that, let's not break people's importfeed cron jobs, and
importfeed does not typically have to check a large number of new items,
so it's ok if it's a little bit slower when used with youtube playlist
feeds.

importfeed's behavior is also improved (?) when a feed has links in it
to non-media files. Before, those were skipped. Now, the content of the
link is downloaded. This had to be done, because trying to use
youtube-dl is slow, and if those were skipped, it would have to check
every time importfeed was run. While this behavior change may not be
desirable for some feeds, that intersperse links to web pages with
enclosures, it will be desirable for other feeds, that have
non-enclosure directy links to media files.

Remove old quvi modules.

This commit was sponsored by Øyvind Andersen Holm.

BuildInfo.hs

@@ -81,7 +81,6 @@ buildFlags = filter (not . null)
 	-- Always enabled now, but users may be used to seeing these flags
 	-- listed.
 	, "Feeds"
-	, "Quvi"
 	]
 
 -- Not a complete list, let alone a listing transitive deps, but only