[openstreetmap-website] add details in description meta tag; and add og:description and twitter:description (#1066)

[Paul Norman]

`og:image` is a required property. Given we don't have a way to generate an image specific to the graph object, for the same reasons as #739, what could we do here?

There is the `og:locale` property, so we can do `<meta property="og:locale" content="en_GB" />` It also allows for `og:locale:alternate`, to specify alternate locales which are available, but doesn't talk about how to retrieve those alternate locales.

https://developers.facebook.com/docs/opengraph/guides/internationalization#bestpractices discusses OG localization in the context of Facebook, and a `X-Facebook-Locale` header is used.

> and add og:description and twitter:description

I can't find a reference to `twitter:description` on the twitter graph docs - could you provide a link to documentation?
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@mikelmaron Could you provide what you think the open graph properties would be for a few example objects?

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