Better handling of uploads of many (100+) large files.

[supermoos]

When uploading a folder of 100+ files (probably less too) with files 5-300 mb, it seems like the upload progress stalls a bit because it starts all of them at once. Could we perhaps set a customisable limit of 10 active uploads at a time? Might make it more reliable? Or look into using something like uppy.js (has s3 integration) to upload instead?

[supermoos]

This is for the v2-alpha branch.