Fix #867: Added a SUBSONIC_DEFAULT_TRANSCODING_FORMAT env var to support clients that don't provide the format parameter

Resolve "Clicking on playlist name from PlaylistModal does nothing if current page is that playlist's page"

Fix #830: Better handling of follow/accept messages to avoid and recover from desync between instances

Fix #550: Show remaining storage space during import and prevent file upload if not enough space is remaining

Fix #809: Added admin options to disable login for users, ensure related content is deleted when deleting a user account

- use non_field_errors struct when writing duplicate track errors
- generalize frontend error handler and update frontend error parsing