upgrade jquery-file-upload to 9.5.7
fix so cover url returns placeholder and 404 properly
be smarter when choosing which "other file" belongs to which audio file
upload improvements, better other file handling and other stuff

use a separate "start" upload session call to avoid freezing if first
file is a other file.

always place other files last to avoid freezing if first 4 files are
other files (we just do 4 uploads in parallel)

add a artist_name_fallback per file upload selection to ease fixing tags
if they're missing