move onclick to js file

js/main.js

@@ -10,10 +10,10 @@ function toggle_gototop() {
   }
 }
 
-function gototop() {
+document.getElementById("gototop").addEventListener("click", function(event) {
   document.body.scrollTop = 0; // For Safari
   document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
-}
+});
 
 document.addEventListener('DOMContentLoaded', function() {
   const $navbarBurgers = Array.prototype.slice.call(document.querySelectorAll('.navbar-burger'), 0);