Check for excessively large uploads before they begin

The onHeaders callback is supposed to be able to stop large uploads before
they begin, but do not appear to be having the desired effect. The browsers
continue uploading until completion, before noticing the response. To work
around this for now, upload the sizes explicitly when the job begins and
let Shiny pre-emptively reject the whole thing. This is also beneficial
in cases where multiple files are being uploaded and not all of them
exceed the maximum upload size.