lib helpers - link builder cleanups

`subnav_link()` is only used in legacy templates - these could be replaced by `nav_link()` and `subnav_link()` deprecated.

`nav_named_link()` only used in templates_legacy/layout_base.html - could be replaced with `build_nav()` params need reordering - and deprecate `nav_named_link()`   It is used by `_make_menu_item()` but that should call `_link_to()` directly.

This would reduce the number of these functions from 6 to 3 if we include already deprecated `subnav_named_route()`