support zip for output of transcription service

[marjisound]

this is as a result of a change in transcription service in https://github.com/guardian/transcription-service/pull/122

What does this change?

How to test

How can we measure success?

Have we considered potential risks?

Images

Accessibility

• [ ] Tested with screen reader
• [ ] Navigable with keyboard
• [ ] Colour contrast passed
• [ ] The change doesn't use only colour to convey meaning