Add Canonical tag to meta tags

[MarkoSulamagi]

Cachet pages are often publicly available so it would be useful to have a canonical link in the meta tags. This is useful for SEO (even if there's only 1 version for each page and there're no duplicates).
https://moz.com/learn/seo/canonicalization

If you agree then I could create a PR for this myself.

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[jbrooksuk]

Yep! Agreed. I’d love a PR :)