Postgres back-end

• Version 1.0 🎉
• Postgres backend:
  • isaric schema name
  • isaric_admin read/write role
  • drops known data linkage tables before doing a new release
    • database versioning with pg_dump exports
  • Renaming WP5 RNA-seq report columns - Postgres didn't like some of them
  • New environment variables: PGPASSFILE (optional), ISARIC_LINKAGE_CREDENTIALS
  • New CI system
    • Docker executor
• Generic SQLite/Postgres-compatible function for day comparisons
• Lazy-loading configs
• Cleaning up circular imports in index table queries
• Assays done report:
  • ported to SQLAlchemy
  • sample_assays is now a SQL table, not a WITH statement
• New Conda dependency: postgresql=12.3 (client binaries)
• Closes #10 (closed)