Use a ThreadPoolExecutor to concurrently store files during import (#1427)
Use a ThreadPoolExecutor to concurrently store files during import (#1427)

* Use a `ThreadPoolExecutor` to concurrently store files during import
isislab/CTFd,isislab/CTFd,ajvpot/CTFd,LosFuzzys/CTFd,ajvpot/CTFd,ajvpot/CTFd,CTFd/CTFd,LosFuzzys/CTFd,CTFd/CTFd,CTFd/CTFd,ajvpot/CTFd,CTFd/CTFd,LosFuzzys/CTFd,isislab/CTFd,isislab/CTFd,LosFuzzys/CTFd