OC-720: Email preview improvements

[finlay-jisc]

The purpose of this PR was to improve the appearance of email previews in email client applications.

For AC 1 the best I could do is to have the name followed by the address, i.e. "Octopus.ac noreply@prod.octopus.ac".
For AC 2 I found a way of setting the body preview / snippet but won't be able to test this properly until the code is deployed to int as we can't send emails locally. The preview text values have been agreed with the PO.

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Acceptance Criteria:

1. The email’s sender should be “Octopus.ac” instead of the email address itself.
2. Emails from Octopus display the first line of the email body’s text in the inbox preview, instead of the URL to an image.

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Checklist:

• Local manual testing conducted
• Automated tests added
• Documentation updated

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Tests:

API:

E2E: