[Extra] Step 6. Add wagtail-localize'd sitemap URLs for every page

[KalobTaulien]

If you go to http://localhost:8000/sitemap.xml with more than one locale, it only shows /en/ pages. But it should show several more.

We can add get_sitemap_urls() to our FoundationMeta Mixin and add list items to the returned data. Wagtail core code here.

Steps to reproduce problem

1. inv new-db
2. Sign into the admin
3. Create a new locale and sync them all from the English pages we have
4. Go to http://localhost:8000/sitemap.xml and notice there are no non-english pages (routes prefixed with /en/ but not with anything else)

Acceptance criteria

• sitemap.xml needs to have all the available links to each page for every locale
• Check that sitemap.xml url is cached quite heavily. (May already be done)