fix: Rebuild environments when stop serving flags changed

[zachaysan]

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Changes

Add a conditional after save hook to rebuild the documents if the stop serving flags bool is changed.

How did you test this code?

With a debugger first to make sure the task call was going through, and later with two new tests that mocked it.

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 8, 2023 7:26pm
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 8, 2023 7:26pm
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview		Nov 8, 2023 7:26pm

[github-actions]

Uffizzi Preview deployment-40348 was deleted.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a0eefdd) 95.64% compared to head (6a05f92) 95.64%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2944   +/-   ##
=======================================
  Coverage   95.64%   95.64%           
=======================================
  Files        1012     1012           
  Lines       29154    29173   +19     
=======================================
+ Hits        27883    27902   +19     
  Misses       1271     1271           

Files	Coverage Δ
api/organisations/models.py	93.86% <100.00%> (+0.23%)	⬆️
api/organisations/tests/test_models.py	100.00% <100.00%> (ø)

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