feat: support gitlab page meta links

[valdisrigdon]

Fixes #375

An alternative implementation here would be to deprecate the existing github_ fields, rename them to git_ instead of github_ and add a 5th field of git_repo_type where github or gitlab is supported.

I chose to avoid deprecation and add the new fields, but I'm open to implementation suggestions.

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[valdisrigdon]

@chalin ready for a review