Processing overhaul

This PR overhauls the processing system. With the new system, non-critical processing steps can be defined; these will be executed asynchronously and the outcome of their execution will not affect the overall processing status. 

@SirCmpwn - please test locally and let me know before merging - we need to upgrade the server manually.
Also, it'd be nice to have the UI report the `pending` status (i.e, the file is in the queue)