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

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