CCM-14753: approve letter

[alexnuttall]

Description

Adds an approval endpoint and the review-and-approve-letter-template page.

The API route fetches the template first, in order to get the letter variant ID, then the letter variant. The letter variant's maxSheets value must be known to validate the renders before submission. Validation is done against the fetched template rather than in the typical way, in a conditional update. The conditional update itself only uses lockNumber.

Other changes

• Remove an undefined style on details component headings (styles.preview__heading)
• Add proper 'open in new tab' behaviour to a link on the authoring letter preview page

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
• If I have used the 'skip-trivy-package' label I have done so responsibly and in the knowledge that this is being fixed as part of a separate ticket/PR.

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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.