feat: add postfix relay support

[RobHooper]

This PR adds relay support to Postfix. We need this on portland-dev in order to send administrative email as they are being blocked by the ISP.

@jpmckinney , pillar.smtp is already configured on Credere, but I cannot see where it is in use in Salt?
I re-used smtp because it configures the same service this adds to Postfix.
However, if this clashes other parts of the code, we could rename the values in this PR to pillar.postfix.relay.

To Do:

• Create SMTP credentials and add to pillar private (docs)
• Merge and deploy to portland-dev

[jpmckinney]

pillar.smtp is already configured on Credere, but I cannot see where it is in use in Salt?

Ah, it's not used, indeed - we configure it again in the same file as AWS_REGION, AWS_ACCESS_KEY, AWS_CLIENT_SECRET. The credentials were the first thing I added, before the app had any environment variables.

I've removed it now.

[jpmckinney]

Create SMTP credentials and add to pillar private (docs)

I think we can reuse the credentials in smtp.sls?