Add some more useful default fields

[Fishbowler]

The extended info is defined in https://xmpp.org/extensions/xep-0045.html#disco-roominfo
https://xmpp.org/extensions/xep-0045.html#example-10 has a bunch of examples of information that might be returned.

This looks useful and could be easy to grab:

      <field var='muc#roominfo_changesubject'
             label='Occupants May Change the Subject'>
        <value>true</value>
      </field>

Is there more config we could surface too?