Send XMPP invitation when user is invited to a room

[ge0rg]

When you invite a bridged xmpp user to a matrix room, the xmpp user should receive a room invitation from the room's JID according to XEP-0045 §7.8.2:

<message
    from='#room#domain@matrix.org'
    id='nzd143v8'
    to='bare-user@domain'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <invite from='user_domain@matrix.org'>
      <reason>
        Invitation text
      </reason>
    </invite>
    <password>do-you-have-passwords?this-is-optional</password>
  </x>
</message>

[Neustradamus]

@Half-Shot: What is your progress on it?