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!
Does the implementation of this method throw any exception? (What I assume :see_no_evil: )
We should add the phpdoc accordingly