Allow users to import private videos

Right now, if you have a private video and you enter the key id as the video identifier, things almost work: the video metadata are correctly imported, and the thumbnail is imported too. During the import process though, the entered key is lost, and the importer falls back to the video public identifier ; at that point, showing the video thus doesn't work.

Attaching a patch soonish that'll preserve the entered key for private videos.