mqtt: Add Home Assistant config for refresh button

The MQTT Marionette will now publish Home Assistant discovery
configuration for a button entity to initiate a page reload.  Each
managed screen has its own button, so individual windows are refreshed
independently.

src/mqtt.rs

@@ -285,6 +285,27 @@ impl<'a> MqttPublisher<'a> {
         trace!("Publishing message: {:?}", msg);
         self.client.lock().await.publish(msg).await?;
 
+        let command_topic = Some(format!("{}/{}/refresh", prefix, screen));
+        let name = format!("Refresh {}", screen);
+        let unique_id = format!("button.{}_refresh", key);
+        let object_id = unique_id.clone();
+        let config = HassConfig {
+            command_topic,
+            name,
+            unique_id,
+            object_id,
+            state_topic: "".into(),
+            icon: "mdi:refresh".into(),
+            ..config
+        };
+        let msg = Message::new_retained(
+            format!("homeassistant/button/{}_refresh/config", key),
+            serde_json::to_string(&config).unwrap(),
+            0,
+        );
+        trace!("Publishing message: {:?}", msg);
+        self.client.lock().await.publish(msg).await?;
+
         let unique_id = format!("light.{}", key);
         let object_id = unique_id.clone();
         let command_topic = Some(format!("{}/power", prefix));
@@ -296,6 +317,7 @@ impl<'a> MqttPublisher<'a> {
             name,
             unique_id,
             object_id,
+            icon: "mdi:monitor".into(),
             ..config
         };
         let msg = Message::new_retained(