Reapply and tweak stage validation for site_address_is_company_address field

[oliverjwroberts]

Description of change

Reapplies PR #5928 (commit 51232c9), which also modified the validation for site address fields. These are now only required if site_address_is_company_address is not None.

The only difference is this PR/commit keeps the validation for the actual_uk_regions field.

Updated frontend PR can be found here.

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 96.64%. Comparing base (e762bb1) to head (1e675ae).
Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5939      +/-   ##
==========================================
- Coverage   96.64%   96.64%   -0.01%     
==========================================
  Files        1069     1069              
  Lines       25250    25253       +3     
  Branches     1663     1663              
==========================================
+ Hits        24404    24405       +1     
- Misses        690      691       +1     
- Partials      156      157       +1     

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