[6.x] Fix nav entry links across sites losing their domain

[lazerg]

On a multi-site install with sites on different domains, a nav item linking to an entry on another site rendered {{ url }} without that entry's scheme and host, so the browser resolved it against the current site and 404'd. Page::url() always fell back to the linked entry's relative URL, even when the nav had select_across_sites turned on, so only {{ permalink }} picked up the right domain. The same relative URL fed is_current and is_parent, so a cross-site item could get flagged as active whenever the two sites happened to share a path.

Page::url() and urlWithoutRedirect() now check the nav's select_across_sites setting and return the entry's absolute URL when it's enabled, mirroring the fix already applied to Bard entry links in #12694. AugmentedPage needed its own url()/urlWithoutRedirect() overrides too, since the augmented url key was resolving straight off the underlying entry instead of going through the page.

Fixes#15237