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.

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• Do the end to end tests need updating before these changes will pass?
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