Allow importing as arbitrary MB release id

[lucat1]

Currently, if the music brainz API returns unrelated matches for an import, there is no way for the user to say "i know better" and force the tagger to import the files as another release. The imports API could be extended by allowing a json:api-like insert operation to add a new music brainz release ID. The addition would then be asynchronous and the client would have to wait for its compatibility to be evaluated.