To decomission the old test VM (wmfg-test, IP 83.169.58.57) we need to move the backend-test.wikimedia.de application to the new server

• Add backend nginx config to infrastructure repo
• Copy backend code to wmfg-test2 (or better: write an atomic Ansible deployment playbook like the one for the frontend)
• Switch subdomain IP from 83.169.58.57 to 83.169.58.60

To decomission the old "production" VM (wmfg-test, IP 83.169.58.52) we need to move the staging environment to the new production server, as outlined in T147486