Fixes https://github.com/rails/rails/issues/48477

Using forms inside emails is not common, but it's possible, and it's also possible to share views between controllers and mailers. Currently if a controller sets a `default_form_builder` that's different from the global config, mailers that use the same views will not default to the same `FormBuilder`. To fix this, this PR adds a `default_form_builder` method for mailers that does the same thing as its controller sibling.