Make Panel Chat Interface callback not require user and instance args

[ahuang11]

I find myself typing it excessively, but not using it:

def callback(contents, user, instance):
    return contents

I'd like to make the latter optional by perhaps inspecting the signature to see if it contains it.

def callback(contents):
    return contents

That way, it also conforms a bit more to pn.bind(callback, contents=input.param.value).