Add `path_prefix` and `build_ref` to PagesDeployment

As part of the introduction of the GitLab Pages Multiple Versions Support, we need to add path_prefix and build_ref to PagesDeployment:

• the path_prefix will be used to build the versioned url
• the build_ref (git reference) will be used to clear deployments when the build_ref is deleted (branch/tag)

New pages builds must save the build.ref in the PagesDeployment#ref column.

• PoC app/services/projects/update_pages_service.rb reference