GREG-154: Add gender field

The field is showing when the institution is UiB.

gregui/tests/api/views/test_invitation.py

@@ -93,7 +93,7 @@ def test_get_invited_info_session_okay(
         passport=None,
         feide_id=None,
         date_of_birth=None,
-        gender=None
+        gender=None,
     )
     assert data.get("sponsor") == dict(
         first_name=sponsor_foo_data["first_name"],