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!
We can do that. - I'll then send a mail to the dev-ml about the change once we agreed on this approach.