Footer should link to Rosetta pages when available

[iandunn]

Similar to #49, the footer links need to go to Rosetta-specific pages, when they're available. When they're not, they should fallback to the main site.

e.g., https://nl.wordpress.org/about/ exists, so the current footer uses that. There isn't a translated Hosting page, though, so it just links to https://wordpress.org/hosting/

The simplest way might be to copy the old footer, which just manually makes specific links use $localised_domain.

If that won't work well, then another approach might be to:

• rename get_global_menu_items() to get_global_header_menu_items()
• create a similar function called get_global_footer_menu_items(), populate it w/ the current/English links
• convert footer.php to loop through the items programattically, and generate wp:navigation-link comments, just like header.php does. Maybe create a helper function to make it DRY across both files.
• create a function called something link inject_rosetta_urls(). it could be passed the global items, and loop through them. when the rosetta site has a localized page, it could replace the url/title. then it'd return all the items (some of which are now localized, and some of which are still the original English version)
• in render_global_footer(), do something like:

   $menu_items = get_global_footer_menu_items();
   
   if ( is_rosetta_site() ) {
   	$menu_items = inject_rosetta_urls();
   }

[ryelle]

Now that #106 is merged, is there anything else to do here?

[iandunn]

Nope 👍🏻