Option to set subject and text by message code

[davidkron]

I think it would be useful to have the option to set a mail's subject and text content by providing a message code. As a developer, the advantage is that I don't have to include the messageSource myself and with the already available "locale" option subject and text could be translated to the correct locale.

Code example:

mailService.sendMail {
    locale user.languageForCorrespondence
    subject 'mail.summary.subject'
    text 'mail.summary.text'
}