add SSEs for Field Report updates, listen for them from frontend

[srabraham]

this starts background-refreshing the Field Reports and Field Report pages based on changes to Field Report entities. I still have TODOs in here around making some of that more efficient and on making the incident (and incidents?) page listen for updates as well.

#1498

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 12 lines in your changes missing coverage. Please review.

Project coverage is 69.75%. Comparing base (94d2237) to head (c455393).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ims/application/_eventsource.py	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1523      +/-   ##
==========================================
- Coverage   69.81%   69.75%   -0.07%     
==========================================
  Files         182      182              
  Lines        9167     9184      +17     
  Branches     1508     1511       +3     
==========================================
+ Hits         6400     6406       +6     
- Misses       2657     2668      +11     
  Partials      110      110              

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