Tweak repr of ChatMessage

[ahuang11]

While debugging, the ChatMessage repr, and also the ChatFeed, can be excessively bloated and spammy. This reduces it to show only object, user, and reactions.

After (top) and Before (bottom):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.38%. Comparing base (c6ac0a8) to head (ada9154).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7521       +/-   ##
===========================================
+ Coverage   41.55%   82.38%   +40.83%     
===========================================
  Files         327      339       +12     
  Lines       48679    51007     +2328     
===========================================
+ Hits        20228    42022    +21794     
+ Misses      28451     8985    -19466     

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