Improve SEO/opengraph tags based on hugo builtins

[mattstratton]

Hugo has some internal templates, which I still don't think will work for us, but we can get some good ideas from them.

https://github.com/spf13/hugo/blob/e37e3dcc0465b3834cf47c954b99d3587b324319/tpl/tplimpl/template_embedded.go#L161

[mattstratton]

See also gohugoio/hugo#2979