Have registrations export generate wcif_status instead of single-letter

[dunkOnIT]

Still need to test locally - but I think if we just return the WCIF status in the endpoint we improve the CSV export considerably.

Manual tests:

• [x] Exporting registrations in different states returns the WCIF states in the CSV
• [ ] Importing registrations with different WCIF states adds only accepted registrations (existing behaviour)
• [ ] Importing registrations with "a" states adds them as registrations (backwards compatibility)

[dunkOnIT]

The current plan is to create backwards compatibility for CSV imports by supporting either the letter or the status - that's the current blocker for this PR. I'll also ask around for anyone using the export function, though

[gregorbg]

Kindly pinging this PR

[dunkOnIT]

Pretty low priority for me compared to the Registration Service work, but I may pick this up as part of the WCIF/Registration Service integration I'll be starting soon