Implement support for TranslatableModel.Meta.ordering option.

__Objective:__
Add support for setting both translatable and untranslatable fields on the `ordering` option of translatable models. Before this PR, using this option requires ugly hacks (like ordering on `"master__shared_field"`).

__Method:__
Anticipate when the default model ordering will be used.
Force ordering to a translated version of default model ordering in that case.

__Result:__
Works as expected. Just one catch: if using translated fields on model's `ordering`, any use of a `FallbackQueryset` that uses the ordering must call `order_by`. This is logical as ordering on a translated field does not make sense on `untranslated()` queryset.