Make Device.pubsub_* helpers into proper decorators

[alexdewar]

At some point, these helpers lived outside the Device class and functioned as normal decorators. Now the way they're used is a bit strange, e.g.:

        self._requester.make_request(
            f"http://{OPUS_IP}/opusrs/{filename}",
            self.pubsub_errors(self._on_reply_received),
        )

Despite what I clearly believed in the past, it is possible to have decorators which operate correctly on member functions: https://stackoverflow.com/questions/11731136/class-method-decorator-with-self-arguments

It would be much cleaner if member functions could just be decorated with pubsub_errors and pubsub_broadcast directly.