Render SES emails using markdown

[DavidBiddle]

What problem does this pull request solve?

Trello card: https://trello.com/c/d5t1W2Hs/3151-render-ses-emails-using-markdown

Converts our existing SES email templates into markdown. This means that we no longer have to manually maintain separate HTML and plaintext templates for these emails - instead we can write a single markdown template and render it as HTML and plaintext respectively. It also reduces the number of inline styles we have to maintain.

Screenshots

Submission email

Daily batch email

Weekly batch email

Confirmation email with copy of answers

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• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

[github-actions]

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