Fix active menu trail for extensionless URLs

What does this MR do and why?

The active menu trail does not highlight for all paths where the page can be accessed:

• https://new.docs.gitlab.com/user/gitlab_duo/use_cases/ has trail 👍
• https://new.docs.gitlab.com/user/gitlab_duo/use_cases.html has trail 👍
• https://new.docs.gitlab.com/user/gitlab_duo/use_cases no trail 👎

This MR adjusts the active item logic so that it should work for any of these possible paths.

This also adjusts nav-related tests to better reflect the change to uglyURLs in !139 (merged).

This MR also includes a fix for a typo introduced in !143 (diffs). The new site is new.docs.gitlab.com, not new.gitlab.com 🤦.

How to set up and validate locally

n/a, can't test this locally

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