Add new permission for default administration menu

To remove the default "Manage" tab in the toolbar that opens up Drupal's default Administation menu the module checks if the current user has the administrator role in a hook_toolbar_alter(). This has caused some confusion when the administrator role either wasn't present or wasn't assigned to users who still needed access to that menu as described in #2941338: Turn off for administrator.

Let's change that check to a new permission so it can be assigned to other roles as needed and ensures that the default administration menu isn't removed from user 1.