frontend: no delay after large SRPM upload
    
    Don't upload files to /tmp, but directly to STORAGE_DIR.  That means we
    don't have to do the expensive /tmp => STORAGE_DIR copy once the file is
    uploaded.  In turn clients don't have to wait a little longer (and
    potentially timeout the http request).
    
    Flask report: https://github.com/pallets/flask/issues/4844
    
    Fixes: #2284
    Merges: #2334