[Etherpad] Migrate main instance from MySQL to PostgreSQL

pica-db-backup/config/backup_config.json

pica-etherpad-db/CHANGELOG.md

pica-etherpad-db/Dockerfile

pica-etherpad-db/README.md

pica-etherpad-db/clair-whitelist.yml

pica-etherpad-db/config/binary_log.cnf

pica-etherpad-db/config/myisam_perf.cnf

pica-etherpad-db/docker-compose.yml

pica-etherpad/Dockerfile

pica-etherpad/docker-compose.yml

pica-etherpad/entrypoint.sh

pica-etherpad/secrets/etherpad-db.secrets.example

pica-etherpad/settings.json