Fix new hunt submission to not rely on global python state

Fix new hunt submission to not rely on global python state

It was only by accident that what I had written happened to work,
(apparently AWS reuses Lambda containers within a 30-minute window or
so). Obviously, I don't want to rely on anything like that, so instead
in this commit I take advantage of the "private_metadata" field that
Slack offers for me to pass data back and forth on a view and use that
to decide which function to invoke to handle view submission.