N°2847 - Navigation menu: Minor adjustments

- Adjust some sizes and colors on the navigation menu to match mockups
- Fix menu groups displaying over the user picture when menu is expanded
- Partially fix notification menu positioning
- Extract some sizes into variables for the popover menu

css/backoffice/components/_all.scss

@@ -19,6 +19,6 @@
 @import "breadcrumbs";
 @import "quick-create";
 @import "global-search";
-@import "popover-menu";
-@import "popover-menu-item";
+@import "popover-menu/popover-menu";
+@import "popover-menu/popover-menu-item";
 @import "newsroom-menu";