Reference (nickname) field mandatory for Innovation & PO applications

[Lubosky]

📝 A short description of the changes

Made reference (nickname) field mandatory for Innovation & PO applications. Adding required to the input should be enough for now along with backend validation and should work OK also without JS.

Added also rake task to backfill reference (nickname) for applications where its missing.

To run the task use command bundle exec rake form_answers:populate_nickname

🔗 Link to the relevant story (or stories)

Asana card here: https://app.asana.com/0/1200504523179345/1207631171095194/f

Deployment implications

None

✅ Checklist

• Features that cannot go live are behind a feature flag/env var or specify deploy date and open PR as draft
• I have checked that commit messages make sense and explain the reasoning for each change
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have squashed any unnecessary or part-finished commits

🖼️ Screenshots (if appropriate - no PII/Prod data):