OF-1683: Make functionality configurable
This commit adds a configuration property that allows administrators to enable or disable XEP-0359 defined behavior. By default, the behavior is now enabled, but only for messages
that are sent to a MUC.
OF-1683: Place SID element in child element of IQ.
When placing a XEP-0359-defined element in a stanza, place it inside the child element
of an IQ, instead of next to the child element (which is illegal, as the XMPP spec prohibits
having more than one child element in an IQ stanza).