Sync/update_translation_fields verbosity and app selection

The goal was to make the commands more convenient to use as part of setting up the test database, in a ``post_migrate`` handler ([an example](https://github.com/wrwrwr/mezzanine/blob/388256dedf38313a94336f5431be9cfb597028e6/mezzanine/utils/tests.py#L67)).

The changes in the first commit make it possible not to display any output from ``sync_translation_fields`` (by passing it ``verbosity=0``). Those from the third commit make it possible to avoid processing all models for each ``post_migrate`` signal sent by specifying a single app for which to do the synchronization / updating (during test database setup the signal is sent once for each installed app). 

The second commit is just a refactor of ``sync_translation_fields``, mostly aesthetic matter; it has [an unsquashed history](https://github.com/wrwrwr/django-modeltranslation/commits/cleanup/sync-translation-fields) -- neither very clean nor really interesting however. There are also [a few things](https://github.com/wrwrwr/django-modeltranslation/compare/wrwrwr:feature/sync-translation-fields-verbosity-and-app-config...cleanup/management-commands-1.8) that have to wait for later.