Refactor SlmMail

Hej @juriansluiman,

I started to think about SlmMail.

Basically, I've removed all the Di stuff, all the message, services... As I told you some weeks ago, I think it makes no sense to have the service send the message. To my opinion this is transport role.

I think a lot of logic could be moved to AbstractHttpTransport, by making each transport filling a kind of "descriptor" (for instance, say that from email is stored in "from_email" in params), so that we don't duplicate the boring stuff accross all transports.

I also think we must separate Transactional emails and Campaign emails. I suppose we should add a "Campaign" namespace into SlmMail to handle Campaign and List (List could extend from AddressList and implements custom logic).