Refactors mail sending code
Replaces fake SMTP server implementation in handler spec

- NotifyUser handler now receives a mail.ClientInterface
- spec uses a fake instead of the real mail.Client
Refactors UAA client usage in NotifyUser

- UAA client is now injected through the constructor as an interface
- UAA client in the test for NotifyUser is replaced with a fake
Returns an unavailable status message when the SMTP server is down

[#73644724]