Allow uplaoding to begin before any other part of the video creation workflow

[bedeho]

This is actually standard on otehr platforms, but we have not changed this in the new server- assited upload version of Orion. I believe this will result in more invasive changes to the API and to Atlas studio, but the UX here is quite superior imho. We should at least try to map out how much more complicated this would be, both in Orion and Atlas.