Add export wins status totals

[elcct]

Description of change

This adds export win status totals (counts for confirmed - customer_response__agree_with_win filter). It adds the following to the list response:

{
    ...
    "confirmed": {
        "null": <count of agree_with_win == null>,
        "true": <count of agree_with_win == true>,
        "false": <count of agree_with_win == false>
    }
}

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 96.65%. Comparing base (95c1713) to head (6ca72f7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5919   +/-   ##
=======================================
  Coverage   96.65%   96.65%           
=======================================
  Files        1076     1077    +1     
  Lines       25320    25329    +9     
  Branches     1669     1669           
=======================================
+ Hits        24474    24483    +9     
  Misses        690      690           
  Partials      156      156           

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