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* Added the ability for all menu tags to (attempt to) identify ancestor pages
and section root page by using components from the request path when serving
a custom URL (not routed via the page tree / served by `wagtail_serve`)
* Added `modify_submenu_items()` method to `MenuPage` model, which takes
responsibility for modifying the initial menu_items list in `section_menu`
and `sub_menu` tags. A DRYer approach, that is easier to extend/override to
meet custom needs.