"Kiosk mode" is a "read-only" Zulip suitable for embedding into
an iframe on another site. I say "read-only" in quotation marks,
because the account is still a fully-fledged active account on
the server, and we just tear out a bunch of stuff in Javascript
(that a malicious user could easily re-enable).
So in that sense, it's not actually safe in security-sensitive
environments -- malicious users logged in via kiosk mode
can do anything the kiosk-mode user can do.
(We need this functionality for the customer3 realm specifically;
we'll possibly just tear this code back out once that experiment
has run its course.)
(imported from commit deb035b4c702fcdb0e660ed549fe74c682abb6d9)
