discogs: set media on tracks (#1921)
discogs: set media on tracks (#1921)

Set the media for all the tracks when fetching a release using discogs,
as autotag.apply_metadata requires it in order to work properly.

jackwilsdon/beets,mosesfistos1/beetbox,mosesfistos1/beetbox,shamangeorge/beets,SusannaMaria/beets,jackwilsdon/beets,xsteadfastx/beets,ibmibmibm/beets,beetbox/beets,pkess/beets,Kraymer/beets,SusannaMaria/beets,madmouser1/beets,SusannaMaria/beets,diego-plan9/beets,beetbox/beets,lengtche/beets,sampsyo/beets,ibmibmibm/beets,lengtche/beets,Kraymer/beets,artemutin/beets,MyTunesFreeMusic/privacy-policy,beetbox/beets,xsteadfastx/beets,diego-plan9/beets,pkess/beets,Kraymer/beets,madmouser1/beets,ibmibmibm/beets,MyTunesFreeMusic/privacy-policy,lengtche/beets,madmouser1/beets,sampsyo/beets,artemutin/beets,shamangeorge/beets,mosesfistos1/beetbox,lengtche/beets,artemutin/beets,mosesfistos1/beetbox,jackwilsdon/beets,pkess/beets,artemutin/beets,ibmibmibm/beets,xsteadfastx/beets,diego-plan9/beets,shamangeorge/beets,sampsyo/beets,sampsyo/beets,MyTunesFreeMusic/privacy-policy,SusannaMaria/beets,Kraymer/beets,jackwilsdon/beets,madmouser1/beets,MyTunesFreeMusic/privacy-policy,diego-plan9/beets,pkess/beets,xsteadfastx/beets,beetbox/beets,shamangeorge/beets