Always use ugettext_lazy for model definitions

As discovered in #1003, some fields of ModelForms were not translated.
The issue is that we've used ugettext instead of ugettext_lazy. The
former will be translated whenever model definitions are loaded, whereas
the latter will translate at the last possible moment. Only then do we
know of the appropriate language to use. Hence, we must use
ugettext_lazy for all model defintions.