Handle theguardian.com short url correctly

[buck06191]

What does this change?

CAPI has been updated to use www.theguardian.com for short urls. I've added the appropriate regex here so that the facia client correctly removes this when getting the discussionId and added tests to check for this.

Related PRs:

• https://github.com/guardian/frontend/pull/23565/files Temporary fix in Frontend that this supercedes
• https://github.com/guardian/content-api/pull/2547 Explanation for why I'm checking for www.
• https://github.com/guardian/frontend/pull/23337 Initial handling in Frontend

  How to test

  sbt test

How can we measure success?

Correct discussion ID passed through from new short URLs.

Have we considered potential risks?

The CAPI change has already occurred so this shouldn't be a breaking change. Even so, I've kept the handling of the old gu.com domain in.