tpl: Improve generation of OpenGraph date tags
tpl: Improve generation of OpenGraph date tags

Allow all nodes/pages to use date-related OpenGraph meta tags, not
only sites which are using the as-of-yet unfinished .Site.Authors
functionality.

Improve compliance of tags with Facebook's OpenGraph docs for the
"website" and "article" types[0][1]. Also, use the proper tag for
modification date (og:update_time vs article:modified_time).

Generate date published using either .PublishDate or .Date, and use
.Lastmod for modification date, which can use the new enableGitInfo
functionality from Hugo 0.18, but seamlessly falls back to .Date if
the site does not have this enabled/supported.

[0] https://developers.facebook.com/docs/reference/opengraph/object-type/website/
[1] https://developers.facebook.com/docs/reference/opengraph/object-type/article/

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