Fix redirects in GenericIE and use it to simplify valid url regex in YoutubeUserIE

YoutubeUserIE's valid url regex is too generic.
It matches youtube.com/watch_videos?... URLs. These URLs get redirected (GenericIE) to URLs that are handled by YoutubePlaylistIE.
URLs of the form youtube.com/USERNAME get redirected to youtube.com/user/USERNAME so it's unnecessary to match the first form in the valid url regex.