Fix/dropdown accessibility (#4977) (#4978)
* Allow id and aria-controls props to pass through AbstractNavItem if navContext doesn't provide a value. * Fix documentation typo * Add aria-controls to NavDropdown and always render the dropdown menu. * Add tests for the dropdown id and aria-controls changes * Add aria-controls to DropdownToggle in DropdownButton and SplitButton * Add tests for aria-controls attribute on DropdownButton and SplitButton * Fix test name typo * Add tests for aria-controls attribute on DropdownButton and SplitButton * Add prop to render dropdown menus in the DOM before being shown * Display a warning if user's id is ignored in AbstractNavItem. Also pass the user provided id through to Tab children so the inner AbstractNavItem knows the user's id is being overwritten.