frontend: handle UTF-8 characters in GraphViz graphs

frontend/assets/templates/bgpmap.tpl

@@ -5,8 +5,19 @@
 <script src="/static/jsdelivr/npm/viz.js@2.1.2/viz.min.js" crossorigin="anonymous"></script>
 <script src="/static/jsdelivr/npm/viz.js@2.1.2/lite.render.js" crossorigin="anonymous"></script>
 <script>
+  function decodeBase64(base64) {
+    const text = atob(base64);
+    const length = text.length;
+    const bytes = new Uint8Array(length);
+    for (let i = 0; i < length; i++) {
+        bytes[i] = text.charCodeAt(i);
+    }
+    const decoder = new TextDecoder();
+    return decoder.decode(bytes);
+  }
+
   var viz = new Viz();
-  viz.renderSVGElement(atob({{ .Result }}))
+  viz.renderSVGElement(decodeBase64({{ .Result }}))
   .then(element => {
     document.getElementById("bgpmap").appendChild(element);
   })