A major server upgrade

I refactored the whole process of additional watermarking required by the artist for anti-piracy purposes.

Now if the artwork is marked for additional watermarking, visitors will be displayed a progress bar to manually track the progress of watermarking.

And, if the artwork is marked for additional watermarking, zip generating task can only be requested after all watermarking tasks are done successfully.

This refactoring can make sure contents are not broken before they are presented to you.