[DNM] Set response headers outside of Next middleware

[amoore108]

Summary

Adds logic to set response headers outside of Next middleware in the event that we need to upgrade Next without a fix in place for vercel/next.js#86303

Code Changes

• Moves localhost rewrite of the sw.js file from middleware to next.config as it will be known at build time where its location is
• Should behave exactly the same as it does when it was in Middleware. This is just a small optimisation to remove that processing from Middleware
• Changes isLocal check in next.config to check against SIMORGH_APP_ENV instead of SIMORGH_PUBLIC_STATIC_ASSETS_ORIGIN
• Adds cspHeaderResponseForNextDocumentContext function specifically for use in the _document file. This is mostly the same as the cspHeaderResponse function in the same file, but handles header setting a bit differently
• Adds logic to _document to set the header values on getInitialProps

Developer Checklist

• UX
  • UX Criteria met (visual UX & screenreader UX)
• Accessibility
  • Accessibility Acceptance Criteria met
  • Accessibility swarm completed
  • Component Health updated
  • P1 accessibility bugs resolved
  • P2/P3 accessibility bugs planned (if not resolved)
• Security
  • Security issues addressed
  • Threat Model updated
• Documentation
  • Docs updated (runbook, READMEs)
• Testing
  • Feature tested on relevant environments
• Comms
  • Relevant parties notified of changes

Testing

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• Manual Testing complete?
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