Migrate to SwiftMail

Replaces the OC_Mail and phpmailer with SwiftMail allowing us to mock it properly.

Fixes the unit test execution on master on local machines and https://github.com/owncloud/core/issues/12014

Conflicts:
	3rdparty
	lib/private/server.php
	lib/public/iservercontainer.php
	tests/lib/mail.php
	tests/settings/controller/mailsettingscontrollertest.php