mod_muc_moderation: Stamp XEP-0421 occupant-id for the acting moderator Gives clients some hint about which moderator it was who did the deed. The @by attribute does have the nick of the actor, but they could change their nickname at some point, which is what occupant-id solves. Ref #1816

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.