Add Legacy win migration function.

[elcct]

Description of change

This adds a function that will let us migrate the legacy Export wins.

The function will be run manually to assess the quality of migration and likely there will be more PRs coming with improvements.

The imported Legacy wins will not be visible to the users without a feature flag.

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.

[code-review-doctor]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.91304% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 96.35%. Comparing base (48be494) to head (de9d982).

❗ Current head de9d982 differs from pull request most recent head efd0080. Consider uploading reports for the commit efd0080 to get more accurate results

Files	Patch %	Lines
datahub/export_win/legacy_migration.py	92.09%	9 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5395      +/-   ##
==========================================
- Coverage   96.38%   96.35%   -0.03%     
==========================================
  Files         931      935       +4     
  Lines       22075    22300     +225     
  Branches     1971     2002      +31     
==========================================
+ Hits        21277    21488     +211     
- Misses        653      662       +9     
- Partials      145      150       +5     

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