always assign default roles on login

successful authentication of a user always grants 'user' role

rather than only on first user creation in db

jupyterhub/handlers/base.py

@@ -771,8 +771,9 @@ class BaseHandler(RequestHandler):
         # Only set `admin` if the authenticator returned an explicit value.
         if admin is not None and admin != user.admin:
             user.admin = admin
-            roles.assign_default_roles(self.db, entity=user)
-            self.db.commit()
+        # always ensure default roles ('user', 'admin' if admin) are assigned
+        # after a successful login
+        roles.assign_default_roles(self.db, entity=user)
         # always set auth_state and commit,
         # because there could be key-rotation or clearing of previous values
         # going on.