Require uk recipient company before project can move to verify win stage

[oliverjwroberts]

Description of change

Investment projects now require the uk_company field (which indicates the UK recipient company) to be populated before moving to the verify win stage. This PR adds the API part of that functionality.

Link to frontend PR

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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.39%. Comparing base (b48c2a1) to head (aa3271e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5461      +/-   ##
==========================================
+ Coverage   96.38%   96.39%   +0.01%     
==========================================
  Files         951      951              
  Lines       22488    22489       +1     
  Branches     2021     2021              
==========================================
+ Hits        21674    21679       +5     
+ Misses        661      659       -2     
+ Partials      153      151       -2     

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