Improve formatting of email messages in Slack

[reichert621]

Email messages in Slack should be formatted more like... email messages.

This is important because the etiquette around replying to emails is a bit different than replying to a chat message, so it's necessary to make it obvious when a message comes in via email so the person responding can adjust the tone/format of their reply.

We can probably just do something like this:

✉️ From: name@example.com
Subject: This is the subject line
Message content here

(Additional metadata below)

Testing
Please write automated tests and include a screenshot in the PR

Questions, or need help getting started?
Feel free to ask below, or ping us on Slack :)

(You can also check out our CONTRIBUTING.md)