Auto upgrade through JS password prompt

Adds admin pass variables and prompt for cases where the surrounding
context tells us we should prompt.

Co-Authored-By: Joe Blaylock <jrbl@jrbl.org>
Remove password prompt for instructor upgrade

Use essentially the same identification and upgrade strategy, but
require a shared secret rather than prompting for a password.

Also, filter out in-channel mode change messages.

Co-Authored-By: Giulio Gratta <giulio@giuliogratta.com>
Merge pull request #2 from Stanford-Online/jrbl/new_auto_upgrade

Jrbl/new auto upgrade