Set company address country to UK for export enquiries

[oliverjwroberts]

Description of change

Company records that were created from Great export enquiries did not have an address country as that information is not present in the incoming data. This, however, was causing issues when users were attempting to verify these company records with the DnB service: 400 - {"address":{"country":["This field is required."]}}.

To resolve this, it was agreed to set the address country as the UK for all newly created company records; a safe assumption given they are export enquiries.

This PR also adds a management command to modify existing Great export enquiries with associated companies that do not have an address country.

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.50%. Comparing base (166cb85) to head (0276d2e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5842      +/-   ##
==========================================
- Coverage   96.50%   96.50%   -0.01%     
==========================================
  Files        1042     1043       +1     
  Lines       24755    24776      +21     
  Branches     1645     1646       +1     
==========================================
+ Hits        23890    23910      +20     
- Misses        709      710       +1     
  Partials      156      156              

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[DeanElliott96]

Looks good to run and then remove.