Include rake task for testing mail sending

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Twitter: https://twitter.com/kaystrobach/status/712987662618124288

To enable easier and quick troubleshooting of email by GitLab sys admins, we can have a rake task that allows one to send a test email regardless of whether the setup is configured to use postfix, SMTP etc.

We already have an incoming_email task btw

cc/ @DouweM @balameb