Navigation: Show only + icons in overlay menu for new NavBar (#47347)

* Nav: Show overlay icons based on allowed list

* user essentials mob! 🔱

* Navigation: clean up and use new backend prop to show plus icons and
improve visual styling

* Nav: Fix top padding

* refactor to not use showIconInNavbar in NavBarMenuItem

* remove a missed bit

* refactor icon into const

Co-authored-by: Ashley Harrison <ashley.harrison@grafana.com>

packages/grafana-data/src/types/navModel.ts

@@ -30,6 +30,7 @@ export interface NavModelItem extends NavLinkDTO {
   highlightId?: string;
   tabSuffix?: ComponentType<{ className?: string }>;
   hideFromNavbar?: boolean;
+  showIconInNavbar?: boolean;
 }
 
 export enum NavSection {