feat: decouple from mailman2 - explicitly model nonwg mailing lists

[rjsparks]

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[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (c64b1da) 88.92% compared to head (a393aa7) 88.94%.
Report is 17 commits behind head on main.

Files	Patch %	Lines
ietf/sync/views.py	8.33%	11 Missing ⚠️
ietf/mailinglists/models.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7013      +/-   ##
==========================================
+ Coverage   88.92%   88.94%   +0.02%     
==========================================
  Files         289      291       +2     
  Lines       40454    40550      +96     
==========================================
+ Hits        35972    36066      +94     
- Misses       4482     4484       +2     

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