Add email deliverability check

[stevejalim]

Description

Recently, due to external-system changes, we potentially lost the ability to send email from Birdbox, despite Birdbox itself not erroring because the issue was downstream at the delivery end.

We can keep an eye on this by using Dead Man's Snitch, which has an email check.

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Success Criteria

• Where email credentials are present, an email is sent every X hours (suggestion: 12) to DMS to confirm outbound email is OK.
• Where email credentials are not present, no attempt to send an email is made.