Implement a new dropdown menu component that will replace all drop-down
menus in the sidebar and is also intended to be used in the LMS activity
views in future.

The menu consists of several components:

 - `Menu` is the dropdown menu itself, with a toggle button and menu
   content
 - `MenuSection` is a group of menu items with an optional heading
 - `MenuItem` is a menu item with a label, optional icon and optional
   submenu with additional actions. The submenu is going to be used for
   auxilliary links and actions related to groups in the groups menu