Fix header url param to not render html

- PR #346 submitted by Eric to the original repo fixes xss bug

app/client/src/js/index.ts

@@ -208,7 +208,7 @@ socket.on('headerBackground', (data: string) => {
 
 socket.on('header', (data: string) => {
   if (data) {
-    header.innerHTML = data;
+    header.textContent = data;
     header.style.display = 'block';
     // header is 19px and footer is 19px, recaculate new terminal-container and resize
     terminalContainer.style.height = 'calc(100% - 38px)';