Fix for homepage preview URLs on proper domains

[lukasbestle]

Description

Summary of changes

• Clean up today's preview token quickfix
• Add unit tests to prevent this bug from reappearing

Reasoning

Sites on real domains have their homepage on https://example.com and not https://example.com/. The unit tests did not catch this because they all used / as site index URL, which naturally does not have this trailing slash problem.

Additional context

Today's quickfix by Basti passes all those tests as well. 🎉

So I think releasing a beta 2 with this PR isn't urgent at all. I decided to change the code mainly for readability and future robustness (Basti's fix only works because the Uri class normalizes the trailing slash as well, but we can't rely on that here).

Changelog

Refactoring (since beta 1, not for final release notes)

• Update preview token logic to reliably fix tokens for the homepage

Docs

None

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add lab and/or sandbox examples (wherever helpful)
• Add changes & docs to release notes draft in Notion