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What's in this PR?

Things to do in this PR:

• [ ] tests for all functions in CodeCorps.Emails
• [ ] couple of remaining dialyzer errors. I'm not sure they're fixable as is. Might need to ignore

References

Fixes #1331

Issues to add before merge

• Make template creation/updating idempotent - Single command either updates or creates depending on the template existing or not. - #1334
• Switch reply to conversation to support multiple recipients - #1335
• Make sure all emails use the new name fallback provided by #1306 - #1336
• Clean up CodeCorps.Emails.Transmissions.Receipt - all with failure points should go into the event handler performing the email send. Receipt.build should not have a failure point. - #1337