Add management command to set site_address_is_company_address for existing projects

[oliverjwroberts]

Description of change

This PR adds a management command to set site_address_is_company_address to a sensible default for existing projects. This will avoid the majority of users having to re-visit the question in the frontend if they've previously answered the site_decided question.

Business rules are as follows for projects that have:

• site_address_is_company_address == None and site_decided == True: set site_address_is_company_address to False.
• site_address_is_company_address == None and site_decided == False/None: leave site_address_is_company_address as None.

Once this management command has been run once in production, it shall be removed.

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 (ce7c202) to head (f822b4f).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5943      +/-   ##
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- Coverage   96.65%   96.64%   -0.01%     
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  Files        1070     1071       +1     
  Lines       25262    25278      +16     
  Branches     1664     1665       +1     
==========================================
+ Hits        24416    24431      +15     
- Misses        690      691       +1     
  Partials      156      156              

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