Auto-start camera for QR code and use back camera

aleksis/apps/paweljong/static/js/paweljong/checkpoint.js

+const scanner = new Html5Qrcode("reader");
+
 function getCheckpointCoords() {
     navigator.geolocation.getCurrentPosition(setCheckpointCoords);
 }
@@ -12,6 +14,7 @@ function setCheckpointCoords(position) {
 }
 
 function onScanSuccess(decodedText, decodedResult) {
+    scanner.stop();
     $("[name='username']").val(decodedText);
     $("[name='username']").closest("form").submit();
 }
@@ -25,6 +28,8 @@ $(document).ready(function($) {
         getCheckpointCoords();
     }
 
-    var scanner = new Html5QrcodeScanner("reader", { fps: 10, qrbox: {width: 250, height: 250} }, false);
-    scanner.render(onScanSuccess, onScanError);
+    let cameraConfig = { facingMode: "environment" };
+    let scannerConfig = { fps: 10, qrbox: {width: 250, height: 250} };
+
+    scanner.start(cameraConfig, scannerConfig, onScanSuccess);
 });