Fix: Accessibility fixes for WCAG 2.2 update

[phil-l-brockwell]

📝 A short description of the changes

• This PR addresses accessibility issues raised in an audit undertaken to meet WCAG 2.2 AA standard. The fixes include adding or linking missing form control labels, adding missing headers, adjusting colours to meet minimum contrast requirements, adding missing language meta tags and adding role tags to menu items.

🔗 Link to the relevant story (or stories)

• Fix checkboxes on /admin/users/new
• Fix activated/deactivated colour codes
• Fix colour contrast on the dev and staging banners
• Add language meta tag 404.html
• Add missing header to Assessor::Reports::Index
• Add role tags to role menus
• Remove duplicate labels on admin/users/new
• Remove label on admin/form_answer#show
• Fix broken aria menus on admin/form_answer#show
• Add or associate labels to inputs on admin/form_answer#show and assessor/form_answers#index
• Add a table header to the applications table on assessors/form_answers#index

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