Migrate OC_Mail to SwiftMailer

This refactors OC_Mail to use SwiftMailer instead of PHPMail, furthermore this fixes the unit tests when running locally due to being able to mock the  mailing class. (#12014)

Requires https://github.com/owncloud/3rdparty/pull/143

@PVince81 @th3fallen @bantu @DeepDiver1975 Please review carefully. - Thanks!
A message that sends itself, hmmm.
From the MVC concepts we'd usually have a model: the message itself and a controller which sends the message.
This means we'd have `IMessage` and `IMessageService` (or `IMailer`).

In theory IMessage could itself be either a plain text message or multipart message, etc. But that might already go too far.
How about having a simple "Message" model class (no IMessage) that contains the message ?
And everything else related to sending the message, the logic, would be moved to "IMailer" implemented as "OC\Mailer"

@icewind1991 @bantu @DeepDiver1975 what do you think ?