Allow connection string to be overridden by environment variable

Remove NullPool left over from using sqlite

database.py

@@ -13,11 +13,12 @@ from sqlalchemy.dialects.postgresql import BYTEA
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import scoped_session, sessionmaker
-from sqlalchemy.pool import NullPool
 from sqlalchemy.sql.expression import cast
 from werkzeug.security import check_password_hash
 
-engine = create_engine("***REMOVED***", poolclass=NullPool)
+DB_URL = os.environ.get("CONNECTION_STRING", "***REMOVED***")
+
+engine = create_engine(DB_URL)
 logging.getLogger("sqlalchemy.engine").setLevel(logging.INFO)
 session_factory = sessionmaker(bind=engine)
 Session = scoped_session(session_factory)