Add the ability to specify additional fields on the user model that can be used for logging in.

tests/test_app/sqlalchemy.py

@@ -32,6 +32,7 @@ def create_app(config, **kwargs):
     class User(db.Model, UserMixin):
         id = db.Column(db.Integer, primary_key=True)
         email = db.Column(db.String(255), unique=True)
+        username = db.Column(db.String(255))
         password = db.Column(db.String(255))
         last_login_at = db.Column(db.DateTime())
         current_login_at = db.Column(db.DateTime())