This is a preliminary commit for upcoming change where we will use
"bot_creation_policy_values" like approach for many other settings where
dropdown represents the only single setting of integer type.
This experimental setting disables sending private messages in Zulip
in a crude way (i.e. users get an error when they try to send one).
It makes no effort to adjust the UI to avoid advertising the idea of
sending private messages.
Fixes#6617.
Pragati Agrawal
Tim Abbott
Matheus Melo
Anders Kaseorg