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* Add ``pluralizable`` and ``python_format`` attributes to ``TemplateMsg`` model and fill them from messages flags;
* Remove ``flags`` attributes from ``TemplateMsg`` model, now the flags are computed from model attributes (``python_format``);
* Add ``plural_message`` attributes to ``TranslationMsg`` model filled when there is plural translation;
* Update core to rightly use these new attributes (when creating/updating catalog);
* Change catalog's translation form to fit with plural messages;
* Add new south migrations for theses changes;