In 9.0, we created a simplified form view for mail templates, in
which the model_id field is retricted to show only the models for
which mass mailing makes sense. However, two of those models were
forgotten. There should have been some sort of override in their
respective modules, but considering the nature of the domain
attributes, there is no easy way to write such overrides, which is
probably why there wasn't any override to begin with. We decided
with tde and al to just add these models in mass_mailing, thus
breaking module abstraction, but avoiding a very complex override
mechanism for a rather simple case like this.