Converts title input to a dropdown in contact details

[TheDancingClown]

📝 A short description of the changes

• For head of business and press contact details KAO wish to force the user to select from an approved list of titles. We first add the list as a constant to the User model in case we wish to validate User titles in the future.
• We add a condition to the sub_fields_question view to render a select if the :title sub_field_key is present.
• We add a placeholder which will be selected if the title is not found in the list. The placeholder option is disabled to prevent users selecting.
• We changed the input field for admin head of organisation and press contact details to a select and include a blank option for when the title is missing.

🔗 Link to the relevant story (or stories)

• https://app.asana.com/0/1200504523179345/1207337151195641

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):