Remove custom model metaclass

[spectras]

This PR moves the code that customizes TranslatableModel subclasses from the TranslatableModelBase metaclass into a signal handler, connected to Django's builtin class_prepared. It gets triggered almost at the same point, but does not conflict with other metaclass-using apps.
We are 100% sure it gets registered in time as it is in the same module as TranslatableModel.

Here is the detailed list of changes:

• empty out TranslatableModelBase and mark it as deprecated. Testcase checks that nothing is broken and a warning is thrown.
• move its __new__ method to a prepare_translatable_model signal handler.
• clean up the signal handler code: running a bit later, it needs less introspection to gather required data.
• force _base_manager to never be an instance of TranslationManager or a subclass. This was actually missing from the metaclass code (*)
• move the associated contribute_translations out of the model, next to the signal handler. It has no business inside the model class and polluted its namespace.

Tests pass. No behavior change was expected.

(*) it has no influence as _base_manager would only get overriden when using use_for_related_fields = True, and this override only breaks if the default_class of the queryset is set to TranslationQueryset. Still, this is the correct behavior.