Draft: Build gitlab-pages Via Nix

What does this MR do?

As this is the simplest golang project we have, I figured it would be a good first attempt to show how easy it can be to take a project and turn it into something built by nix.

I think the big scary build is gitaly, but this was shockingly easy, so I have high hopes.

Part of the explorations surrounding gitlab-development-kit!1822 (closed)

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines