Add a twitter icon to author links ONLY when theme_preview is active

See: https://github.com/GigaOM/gigaom/issues/5440
Because we need to call this stuff in a more custom manner, I've pulled the code from a few functions found in `go-co-authors-plus`.  This line comes from https://github.com/Automattic/Co-Authors-Plus/blob/master/template-tags.php#L213