This commit adds the python_2_unicode_compatible decorator to all models
that define a text representation. Furthermore, it adds a test to ensure
that we never forget to do that.

It is explained in details in the Django docs:
https://docs.djangoproject.com/en/dev/topics/python3/#str-and-unicode-methods

I don't know how I missed them. I think I must've made a mistake while
heavily rebasing this branch; it's the second time I'm surprised to not
find certain changes I'm confident I did.

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.

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