Hover colour for the markdown help button when an inactive tab on the tabbed editor.

Previously, hovering the markdown help button when the editor is on mobile tab mode, it disappeared as it was the same colour as the background. It's now the same colour as the inactive tab text.

core/client/assets/sass/layouts/editor.scss

@@ -121,6 +121,16 @@
 
         }
 
+        @media (max-width: 1000px) {
+            &:not(.active) {
+                .markdown-help {
+                    &:hover:before {
+                        color: #fff;
+                    }
+                }
+            }
+        }
+
         // Give the tab with the .active class the highest z-index
         &.active {
             z-index: 200;