Channel mix adjusments and test
Don't accept youtube music mix urls as playlist
Don't override playlistData to keep getInitialPage()
Remove json constants
Indentation
Move thumbnail id exctraction code to getThumbnailUrlFromId
Add test for "My mix" detection to service tests
Use ITEM_COUNT_UNKNOWN everywhere instead of -1 and add some tests
-New YoutubeMixPlaylistExtractor, that extracts from a mix (auto-generated playlist).
-The url has the format of "youtube.com/watch?v=videoID&playlistID",
where playlistID always starts with "RD" and usually followed by the videoID.
-Change YoutubePlaylistLinkHandlerFactory to create a linkhandler with the given url if it is a mix.
-Change YoutubeService to return YoutubeMixPlaylistExtractor if the url is a mix.