Replaces https://github.com/owncloud/core/pull/19047 and fixes https://github.com/owncloud/enterprise/issues/854  and https://github.com/owncloud/core/issues/19110

This re-adds the debug mode for the mailer which will log the whole SMTP traffic within the log file if enabled.

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

Conflicts:
	3rdparty
	lib/private/mail.php
	lib/private/server.php
	lib/public/iservercontainer.php
	settings/ajax/lostpassword.php
	settings/application.php