Added SwiftMailer Extension to provide an alternate for UserMailer

Uses the AlternateUserMailer hook. Used SwiftMailer third party
library via composer install from wikimedia fork at
https://github.com/wikimedia/mediawiki-core-vendor-swiftmailer/tree/
5.2.0-patch
run $composer update from extension directory to download the
library. Added LICENSE file.

Change-Id: I7de5a4228a34b2d56d5ed8f0d9e48a6066012e41