fix: dropdown menus show over the sidebar

Moved the sidebar to the same z-level as the header, because layout
dictates that it sits above it anyway.

So could move the dropdown menus up to above the sidebar.

src/library-authoring/LibraryAuthoringPage.scss

@@ -12,7 +12,7 @@
 }
 
 .library-authoring-sidebar {
-  z-index: 1001; // to appear over header
+  z-index: 1000; // same as header
   flex: 450px 0 0;
   position: sticky;
   top: 0;
@@ -21,6 +21,10 @@
   overflow-y: auto;
 }
 
+.dropdown-menu {
+  z-index: 1001; // over the sidebar
+}
+
 // Reduce breadcrumb bottom margin
 ol.list-inline {
   margin-bottom: 0;