Adding warn command to warn users blandly

Adding a warn command that warns a specific user using an 
alert box, so he knows he's doing wrong and that he might 
be kicked or worse. This warn is pretty much harmless, but 
it won't kick you out of battle and the moderation action 
message shown everyone in the chat will let everyone 
watching know that that user was doing wrong and that 
they should not do the same to keep being a positive -or a 
not negative- chat presence.
This action is logged in the user's modlog and is thus easy 
to track the punishment scale he is receiving, should he 
keep disregarding the rules, and it's also easy to mark 
troublesome users as warned to be harsher to them upon a new 
bad action on a different day.
Users that are warned can not longer advocate that they didn't 
see the chat -which might be true on some time zones- or that 
they missed the PM.