Don't send a re-connecting player a player-enter event for themself

It's rather ugly that we've got these semantics where the client
doesn't want to see a player-enter event for itself. This is causing
too many special cases in the code (such as the current commit).

So we'll want to change that at some point. But for now we make the
case of a player reconnecting match the semantics. Specifically we
move the add_connection call down _after_ the broadcast of the
player-enter event. This fixes the current client so that it doesn't
display its own name twice when reclaiming a pre-existing player.