CCM-8875 Support pending proof request status

[nicki-nhs]

Description

• Adds enable_proofing feature flag (initially set to false)
• Adds template statusPENDING_PROOF_REQUEST for when files have been submitted and validated
• When template status is PENDING_PROOF_REQUEST:
  • CTA for preview letter template is "Request a proof" instead of "Submit"
  • Status is displayed to the user as "Files uploaded"
• Updates unit tests

Context

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.