refactor: separate concerns / rename notify_events

[jennifer-richards]

Splits the signal receiver logic from the task queuing logic in a way that improves testability. Testing whether a notification email was sent is now just a simple mock to check whether ietf.community.models.notify_of_event() was called with the event.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.73%. Comparing base (6b77807) to head (1f6c0f1).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8328   +/-   ##
=======================================
  Coverage   88.72%   88.73%           
=======================================
  Files         310      312    +2     
  Lines       40882    40892   +10     
=======================================
+ Hits        36273    36285   +12     
+ Misses       4609     4607    -2     

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