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

Splitted admin and other views in dedicated webpack chunks (~25% reduction of chunk size for regular users)

Fix #781: Added a `check_inplace_files` management command to remove purge the database from references to in-place imported files that don't exist on disk anymore