[CMSP-27] Implement sites.yml parsing

[pwtyler]

Scaffolds pantheon.yml and sites.yml validator, with actual implementation for sites.yml.

See fixtures for an annotated example of a valid sites.yml.

[guardrails]

All previously detected findings have been fixed. Good job! 👍🎉

We will keep this comment up-to-date as you go along and notify you of any security issues that we identify.

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👉 Go to the dashboard for detailed results.

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