mod_muc_webchat_url: Don't save templated value This prevents changing of other settings without touching the webchat URL from saving the template value. Otherwise the URL will still be advertised when configuring members-only or hidden.

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.