Gitlab Badges: add project_path_slug to the list of supported variables

The only supported variables/placeholders for badges are: %{project_path}, %{project_id}, %{default_branch}, %{commit_sha}

We would like to have project_path_slug as an additional variable, where forward slashes are replaced by %2F (some-group%2Fsome-project) - this would be helpful for integrating a 3rd party badge.