Re-enable staging site workflow using static-shim

Since the transition from Jenkins to GitLab CI, deployments to www-staging.torproject.org have ceased. Changes are directly pushed to production when merged into the main branch.

This MR is a proposal to move to the same workflow established for blog.torproject.org, whereas changes to the main branch are automatically deployed to the staging environment with a deploy-prod job that needs a manual trigger to push changes to production.