Home Assistant MQTT discovery topics not retained

[mundschenk-at]

Describe the bug
The generated discovery topics are not retained, which means when HA (or simply its MQTT integration) is restarted, they are not there for setting up the entity. HA knows that there was entity previously, so it's still listed in the device view, but it does not have current configuration, so it is unavailable. This is one of the causes for #5.

It also means that #6 never took for me, because previous retained discovery topics created for ICBINV were still around (because non-retained topics don't overwrite retained ones).