Change IRC modes when the user sends an XMPP iq to change some role or affiliation

For example when a user tries to set “affiliation some_nick = owner”, biboumi should send the corresponding «MODE +o » command.

Biboumi should not need to check the rights of the user doing that. Just send the MODE command and the IRC server will return an error if it’s not possible.

Also we need to map “+b” to the “visitor” affiliation.

(from redmine: created on 2015-01-22, closed on 2015-01-24)