Add vertical scrollbar to sidebars that extend outside the viewport

Viewing overflowing sidebar content requires scrolling the entire page which is annoying and causes the reader to lose their place.

Use CSS `calc` to approximate height of the sidebar (`100vh` - height of the masthead) and apply `overflow-y: auto` to add vertical scrollbars when needed.

Fixes #706

_sass/minimal-mistakes/_sidebar.scss

@@ -22,6 +22,12 @@
     &:hover {
       opacity: 1;
     }
+
+    &.sticky {
+      overflow-y: auto;
+      /* calculate height of nav list */
+      height: calc(100vh - 90px - 2em); // viewport height - approx. masthead height - main content top margin
+    }
   }
 
   @include breakpoint($x-large) {