MNTOR-3917: Add plaintext version of Plus expiration email

[Vinnl]

References:

Jira: MNTOR-3917
Figma:

Description

I figured I'd move the plaintext version into a separate PR, since it's not crucial functionality.

How to test

You can view the version in Storybook, and the admin UI will also send both the HTML and plaintext version. Mailinator will let you inspect both.

Checklist (Definition of Done)

• Localization strings (if needed) have been added.
• Commits in this PR are minimal and have descriptive commit messages.
• I've added or updated the relevant sections in readme and/or code comments
• I've added a unit test to test for potential regressions of this bug. - Hmm, I suppose it wouldn't hurt - but I'm going to focus on getting the rest over the line first
• If this PR implements a feature flag or experimentation, I've checked that it still works with the flag both on, and with the flag off.
• If this PR implements a feature flag or experimentation, the Ship Behind Feature Flag status in Jira has been set
• Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• All acceptance criteria are met.
• Jira ticket has been updated (if needed) to match changes made during the development process.
• Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[github-actions]

Preview URL 🚀 : https://blurts-server-pr-5562-mgjlpikfea-uk.a.run.app

[github-actions]

Cleanup completed - database 'blurts-server-pr-5562' destroyed, cloud run service 'blurts-server-pr-5562' destroyed