Parameterise socket preconnect (#1867)

public/index.html

@@ -79,7 +79,7 @@
     />
 
     <link
-      href="https://excalidraw-socket.herokuapp.com/socket.io"
+      href="%REACT_APP_SOCKET_SERVER_URL%/socket.io"
       rel="preconnect"
       crossorigin="anonymous"
     />