PoC: Unique pages site domain

1. Add an option to users to use a unique pages domain, like:
   
   1. The unique domain must be unique
   2. The unique domain creating can be a concatenation of the project's full path and a hash, like: namespace/subgroup/.../project => namespace-subgroup-...-project-1a2b3c4d5e
2. The unique domain must be send in the lookup path internal api /api/v4/internal/pages
3. Gitlab Pages must auto-redirect from the default domain (domain based on the top level namespace) to the unique domain
4. After the validation of the feature the unique domain must be the default behavior