remove required serial number from topic configuration

[madebyTimo]

Removes the serial number input and expects it as part of the topic instead. (#138)
Set the device name as the basename of the topic. Use the whole topic as the device id.

Also removes the leading slash as in #128

[syssi]

Wow! Thanks for your contribution! Is this a breaking change? What happens to people which used the config flow in the past using the old values?

[madebyTimo]

You are totally right. I will see how I can implement a migration. Unfortunately, I'm not very familiar with home-assistant development, so it might take a while.