Fix creation of localised URLs for routable pages

[MaciekBaron]

What is the context of this PR?

This PR relates to a bug described in CMS-1103 which caused the language switcher to not preserve the correct path across domains for all page types.

This PR implements the proposed solution, which preserves the current request path by default and only swaps the language domain or prefix.

This also resolves CMS-765 and CMS-1165.

Specifically, the commit 59150db addresses CMS-1165.

How to review

1. Ensure a page with routable paths exists (for instance an ArticleSeriesPage with /editions)
2. Visit the page
3. Ensure that the language switcher contains the correct URL
4. Trigger a 404 page
5. Ensure that the language switcher is still present

This should be tested for both subdomain and path based locales.

Deployment Safety

Bleed and Sandbox deploy automatically on merge, so PRs should be safe to deploy immediately.

Please select one:

• Safe to auto-deploy
• Not safe to auto-deploy

Follow-up Actions

• Confirm that translated pages will not use a different slug
• Consider how this works with versions (corrections)

[AdamHawtin]

LGTM, working as expected with both path and subdomain based routing locally 👍

[sanjeevz3009]

Did a code review, looks good. Did a functional review locally, and this addresses the change needed from CMS-1103 and CMS-765.

[MaciekBaron]

@MebinAbraham CMS-1165 has been addressed here: 59150db

[BJacksonONS]

Looks good to me, tested 404 pages also with subdomain and path routing.

[MebinAbraham]

As discussed, we'll do the corrections flow changes as a follow up.