[HA discovery] Save the BLE settings and change switch to optimistic false (#1364)

The current way of saving settings is through the retain flag of the MQTT commands, unfortunately if several commands are issues they may overwrite each other. So as to prevent it, we now leverage the memory saving introduced earlier to store the BLE settings.
Also leverage the config status publication to show the state through the button position (optimistic false)

main/ZgatewayBT.ino

@@ -191,7 +191,7 @@ void BTConfig_fromJson(JsonObject& BTdata, bool startup = false) {
     Serial.println();
     return; // Do not try to erase/write/send config at startup
   }
-  pub("/commands/BTtoMQTT/config", jo);
+  pub(subjectBTtoMQTT, jo);
 
   if (BTdata.containsKey("erase") && BTdata["erase"].as<bool>()) {
     // Erase config from NVS (non-volatile storage)