Tech Eval: GitLab Pages without DNS wildcard (NGINX-config version)

Originated from comment.

We want to implement #17584 (closed), and we already have PoC implementing it in pages daemon itself originated from the previous research issue.

However, implementing this inside pages daemon has it's drawbacks:

1. introducing this feature will complicate the codebase a lot and it will require bug fixes every now and then
2. we probably will never use this feature for .com so it will be less reliable because it won't be actively tested until the release goes public

The alternative idea is to implement this feature on the Nginx level inside the omnibus using rewrites.

The goal of this issue is to have PoC implemented on the omnibus level