Allow disabling ScrollToTop button

- refactor corresponding code

usage =>
in site config:
params:
    disableScrollToTop: true

closes: #321

layouts/partials/footer.html

@@ -9,6 +9,7 @@
     <span>&middot;</span>
     <span>Theme <a href="https://git.io/hugopapermod" rel="noopener" target="_blank">PaperMod</a></span>
 </footer>
+{{- if (not .Site.Params.disableScrollToTop) }}
 <a href="#top" aria-label="go to top" title="Go to Top (Alt + G)">
     <button class="top-link" id="top-link" type="button" accesskey="g">
         <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 12 6" fill="currentColor">
@@ -16,6 +17,7 @@
         </svg>
     </button>
 </a>
+{{- end }}
 {{- partial "extend_footer.html" . -}}
 {{- $isHLJSdisabled := (.Site.Params.assets.disableHLJS | default .Params.disableHLJS ) }}
 {{- if (and (eq .Kind "page") (ne .Layout "archives") (ne .Layout "search") (not $isHLJSdisabled)) }}
@@ -34,6 +36,11 @@
             document.getElementById('menu').scrollLeft = localStorage.getItem("menu-scroll-position");
         }
     }
+
+    function menu_on_scroll() {
+        localStorage.setItem("menu-scroll-position", document.getElementById('menu').scrollLeft);
+    }
+
     document.querySelectorAll('a[href^="#"]').forEach(anchor => {
         anchor.addEventListener("click", function (e) {
             e.preventDefault();
@@ -52,6 +59,11 @@
             }
         });
     });
+
+</script>
+
+{{- if (not .Site.Params.disableScrollToTop) }}
+<script>
     var mybutton = document.getElementById("top-link");
     window.onscroll = function () {
         if (document.body.scrollTop > 800 || document.documentElement.scrollTop > 800) {
@@ -63,11 +75,9 @@
         }
     };
 
-    function menu_on_scroll() {
-        localStorage.setItem("menu-scroll-position", document.getElementById('menu').scrollLeft);
-    }
-
 </script>
+{{- end }}
+
 {{- if (not .Site.Params.disableThemeToggle) }}
 <script>
     document.getElementById("theme-toggle").addEventListener("click", () => {