- turned model_variables into a class with lazy loading

- some improvements to Healthchecks

common/models/user.py

@@ -2,7 +2,6 @@ from common.extensions import db
 from flask_security import UserMixin, RoleMixin
 from sqlalchemy.dialects.postgresql import ARRAY
 import sqlalchemy as sa
-from sqlalchemy import CheckConstraint
 
 
 class Tenant(db.Model):