Fixed design nav open/close icon misalignment

no issue

- icons seemed to disappear when sections above are opened, or they lost attachment when scrolling
- added `position: relative` to the nav item container so the `position: absolute` children are correctly contained

ghost/admin/app/styles/layouts/settings.css

@@ -1412,6 +1412,7 @@ p.theme-validation-details {
 
 .gh-nav-design-tab {
     display: flex;
+    position: relative;
     align-items: center;
     box-sizing: border-box;
     width: 100%;