Add a GitHub action CI job to validate 'author' tags

[bstansberry]

News post (and perhaps other content) author metadata values need to match entries in _data/authors.yaml. Add a PR check that it does.

This will avoid problems like what @asoldano fixed for @ehsavoie in #688, and will also help first time authors to realize the need to add an entry for themselves in authors.yaml.

There is no inherent meaning in these values; e.g. they don't need to be github ids. The values in the post files just need to match an entry in the authors.yaml.