PLANET-4811 Add scroll padding

* Prevents the navbar covering the element we're scrolling to. We
probably want to add this site-wide, but probably best handled
distinctly so we can check if this works everywhere.
* For now this padding is tuned for non-logged in users, as the admin
area needs some additional padding which is more tricky to detect. The
scroll-padding-top needs to be on html but on that element we can't know
if we're logged in or not.

assets/src/blocks/Submenu/SubmenuEditor.js

@@ -74,7 +74,7 @@ const renderView = (attributes, setAttributes, className) => {
 
   const { blocks } = useSelect(select => {
     return ({ blocks: select('core/editor').getBlocks() });
-  });
+  }, null);
 
   const flatHeadings = getHeadingsFromBlocks(blocks, attributes.levels);
 

assets/src/blocks/Submenu/SubmenuFrontend.js

@@ -11,7 +11,7 @@ export const SubmenuFrontend = ({ title, className, levels, submenu_style }) =>
     if (!backTop) {
       return;
     }
-    backTop.style.display = 'block';
+    backTop.classList.remove('d-none');
   };
 
   // Enable back top on initial render.

assets/src/styles/blocks/Submenu.scss

@@ -120,6 +120,7 @@ div[data-render="planet4-blocks/submenu"] {
 
 html {
   scroll-behavior: smooth;
+  scroll-padding-top: 4rem;
 }
 
 .back-top,
@@ -134,7 +135,6 @@ div.page-template > .back-top {
   background-color: #074365;
   text-align: center;
   box-shadow: 0 2px 5px rgba(0, 0, 0, .25);
-  display: none;
   z-index: 1000;
 
   @include x-large-and-up {