New unresolver implementation (#9500)
- Move unresolve_domain to unresolver
- Use the new implementation from https://github.com/readthedocs/readthedocs.org/pull/9425 for the unresolver
- Remove settings override from the unresolver, we don't need it.
This doesn't use this new implementation to handle the URLs from proxito nor has support for a custom urlconf, those things will be implemented in another PR.
