Generate a separate short_url for every translation of content.

Some more elegant "nonlocal" Python 2 compatibility fix would be nice.
Started working on tests; for now test that modeltranslation can be
reliably enabled/disabled and that all that should be registered for
translation is registered.

The last two commits (demangling USE_MODELTRANSLATION from USE_I18N
and adding handling for short_urls) were a conclusion from these tests.