Add a "nick" field in the IRC configuration form

What

A "nick" field, in the IRC configuration form (the ad-hoc form of irc.foo.bar@biboumi).

If set, biboumi would ignore any nick passed during the join, and send back the nick in the self-presence. (Maybe there are better ways to do this).

I suppose this doesn't influence the NICK command.

Why

I often want biboumi to set my nick, as I have one that is registered with the server and will rarely change, that might not correspond to my JID localpart nor the default_nick set in my XMPP client.

This often causes me issues because my default_nick is not a valid IRC nick, and the direct translation is taken on some servers already, and I end up getting disconnected.

I can prevent this by setting my nick on every single bookmark, but this quickly becomes tedious.