add ability to download unsupported attachments

[davidjgoss]

🤔 What's changed?

For attachments that aren't a video, image or text, we previously rendered an error message about the attachment being unsupported.

This PR changes the behaviour so we instead render a button to download the attachment as a file. It:

• Works with Base64 or text as the body
• Honours the user-specified fileName from the message if present, generates a filename with the correct extension for the media type if not
• Uses the URL.createObjectURL API, only creating the URL when first requested and cleaning up again when the component is unmounted - so we are doing this in a standards-compliant way without having to externalise a file or crash the browser

It also includes some slight rework in the Gherkin document accordion, so a panel is only rendered when it's expanded - previously this was just hidden by way of display: none. This means we don't render components unless the user is going to see them, and also that any generated download URLs are cleaned up when a panel is closed.

⚡️ What's your motivation?

Fixes #80.

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)
• ⚡ New feature (non-breaking change which adds new behaviour)

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • I have added/updated tests to cover my changes.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• Users should know about my change
  • I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[olleolleolle]

I read the changes commit by commit, and they are very clear and step-by-step. Thanks for doing it that way!

Minor: some console.log statements are perhaps builder's dust? If you think so, sweep it away, otherwise keep it.

Let's get this merged!