new action resetMQTT, called from MQTT Settings page

src/core/mqtt.cpp

@@ -321,8 +321,12 @@ void Mqtt::on_publish(uint16_t packetId) const {
     LOG_DEBUG("Packet %d sent successful", packetId);
 }
 
-// called when MQTT settings have changed via the Web forms
+// called when MQTT settings have changed via the MQTT Settings or Application Settings Web pages
 void Mqtt::reset_mqtt() {
+    if (!enabled()) {
+        return;
+    }
+
     if (!mqttClient_) {
         return;
     }

src/web/WebStatusService.cpp

@@ -195,6 +195,9 @@ void WebStatusService::action(AsyncWebServerRequest * request, JsonVariant json)
         ok = uploadURL(param.c_str());
     } else if (action == "systemStatus" && is_admin) {
         ok = setSystemStatus(param.c_str());
+    } else if (action == "resetMQTT" && is_admin) {
+        emsesp::EMSESP::mqtt_.reset_mqtt();
+        ok = true;
     }
 
 #if defined(EMSESP_STANDALONE) && !defined(EMSESP_UNITY)