Add SEO optimizations

[psolbach]

The current implementation for SEO title isn't flexible enough. site.Title should only be used as fallback for title stored in CF settings. It should be easy in projects to change the SEO title schema ($page.title | site.Title) without the need to overwrite partials/utils/seo/private/get-data.html.

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Addition by @bezoerb from here (#27 (comment)).

(...) we should either only use this scheme when seo_title is not explicitly set, or make it configurable with some kind of variable substitution. Maybe we could use the i18n function for that?

{{ i18n "title_schema" . }}