Improved page preview experience in GitHub

[adaptiva-phil]

First of all, I love the new preview experience in GitHub. It allows our external contributors to view page edits as rendered html without needing to log in. However, there are a couple of improvements that I would recommend:

• Could the previews open in another tab? It seems simple, but when contributors click the preview link it navigates them away from the pull request.
• Could the previews only generate for the space where files have been edited. It could be because we are a mono-repo setup, but the previews get created for every space on the site.