Rename the Playlist and PlaylisEntry models

To avoid confusion around the usage of the term "playlist" and to be consistent with the usage in the Dashboard, we should:

• rename Playlist -> Media,
• rename PlaylistEntry -> MediaSource,
• rename the the custom model permissions, and update their usage,
• rename PlaylistSerializer -> MediaSerializer,
• rename APIPlaylistViewSet - > APIMediaViewSet,
• replace playlist_id -> media_id and update its usage,
• replace default_playlist_id -> default_media_id and update their usage,
• move /api/v1/playlists/ -> /api/v1/media/. The response doesn’t change,
• update the tests.