[#1830] Prevent deletion of last website

[pi-sigma]

Taiga #1830

[stevenbal]

Looks good overall, though I tested this locally and it doesn't work when doing a delete with the "bulk delete" action from the list view (see https://docs.djangoproject.com/en/3.2/ref/contrib/admin/actions/). I think it would be good if deleting the last Site via the list view could be blocked as well

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3743516) 92.84% compared to head (69d688c) 92.85%.
Report is 13 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #843      +/-   ##
===========================================
+ Coverage    92.84%   92.85%   +0.01%     
===========================================
  Files          752      755       +3     
  Lines        26127    26276     +149     
===========================================
+ Hits         24257    24398     +141     
- Misses        1870     1878       +8     

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[stevenbal]

Looks good 👍. I did notice that the delete via the detail page returns both the warning and success message, but since this is an edge case, I don't think it's worth it to override more code just to remove that success message