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!
Done. - It now returns an array with potentially failed recipients. Please be aware that depending on the used mail wrapper this may not be reliable so it should only be used as "nice-to-have" ;-)