Fix name error introduced in c21d845836e4 c21d845836e4 use a ``vocab`` variable but doesn't define it. I assumed ``vocab`` is ``field.vocabulary(form)``.

Fix conditionnal logic broken in 195b24fe3a03 195b24fe3a03 replace: if not req.cnx: by if getattr(req, 'cnx', True): The "not" diseapeared in the process.

[session] provide a simple transaction context manager for session. Closes #1725640

[formwidgets] make the select size a class attribute (more easily tuned)