Add and make use of renamer.scanfolder in downloaders

This is the next step in closing the loop between the downloaders and CPS. The download_id and folder from the downloader are used to find the downloaded files and start the renamer. This is done by adding an additional API call: renamer.scanfolder.

I tested this for SabNZBd only (!) and everything works as expected.

I also added transmission with thanks @manusfreedom for setting this up in f1cf0d91da. @manusfreedom, please check if this works as expected. Note that transmission now has a feature which is not in the other torrent providers: it waits until the seed ratio is met and then removes the torrent. I opened a topic in the forum to discuss how we want to deal with torrents: https://couchpota.to/forum/thread-1704.html