Calling /retro new with a parameter should yell

[pcorpet]

Some of us (@pyduan) tend to forget that /retro new creates a new retro sprint instead of a new retro item, thus resetting the whole list. It would be nice to block a new action when it's sent with additional content /retro new Foo ==> Oops, did you mean "/retro good Foo"?

We could also display the list of the previous retro on a retro new.