Loki: Fix vertical alignment issue in label selector (#18943)

- upgraded rc-cascader (did not fix the alignment issue)
- Safari seems to default to middle alignment, added override to explore
styles
- added empty expand icon property to satisfy API after upgrade

public/sass/pages/_explore.scss

@@ -320,8 +320,7 @@
   z-index: inherit;
 }
 
-// React-component cascade fix: show "loading" even though item can expand
-
+// React-component cascade fix: show "loading" when loading children
 .rc-cascader-menu-item-loading:after {
   position: absolute;
   right: 12px;
@@ -330,6 +329,11 @@
   font-style: italic;
 }
 
+// React-component cascade fix: vertical alignment issue with Safari
+.rc-cascader-menu {
+  vertical-align: top;
+}
+
 // TODO Experimental
 
 .cheat-sheet-item {