Improve markdown TOC scrolling (#349)

Use a better system for scrolling using scrollTo on the
OverlayScrollbars ref. This lets me get the heading as close to the top
of the viewport as possible without the convoluted CSS tricks I was
trying before.

frontend/app/element/markdown.less

@@ -17,7 +17,6 @@
         font-size: 14px;
         overflow-wrap: break-word;
         margin-bottom: 10px;
-        --half-contents-height: 10em;
 
         .heading {
             &:first-of-type {
@@ -26,7 +25,6 @@
             color: var(--app-text-color);
             margin-top: 16px;
             margin-bottom: 8px;
-            scroll-margin-block-end: var(--half-contents-height);
         }
 
         strong {