Add admin menu and data management page

• Add a menu in BiscuIT-ng, labeled, Admin, only shown to superusers (for now)
• Add a view and template for a data management page and add this to the Admin menu
• On the data mangement page, render a completely new menu, called DATA_MANAGEMENT_MENU or whatever
• Have the Untis and SchILD-NRW apps add their menu items to the data management menu
• Remove the main menu links in these two apps