Allow the “On this page” table of contents to scroll

I think the reason `overflow-y: scroll` was applied was to hide the scrollbar (otherwise you end up with the page scrollbar _and_ the TOC scrollbar appearing).

This allows scrolling and hides the scrollbar with `scrollbar-width` in most browsers and `::-webkit-scrollbar` in Safari.

Closes https://bitovi.atlassian.net/browse/LD-199

static/styles/styles.less

@@ -619,7 +619,13 @@ hr:first-of-type {
 .content-nav .list-items {
   flex-grow: 1;
   flex-shrink: 1;
-  overflow: hidden;
+  overflow-x: hidden;
+
+  /* Hide the scrollbar */
+  scrollbar-width: none;
+  &::-webkit-scrollbar {
+    display: none;
+  }
 }
 .sidebar-right .pullout {
   border-bottom: 1px solid #eee;