Refactoring and cleanup of Chat components

[philippjfr]

Make it easier to subclass chat components along with some small fixes.

[codecov]

Codecov Report

Attention: Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.09%. Comparing base (0f46443) to head (9e02c06).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/chat/feed.py	80.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7800      +/-   ##
==========================================
+ Coverage   87.08%   87.09%   +0.01%     
==========================================
  Files         346      346              
  Lines       52970    52962       -8     
==========================================
+ Hits        46128    46129       +1     
+ Misses       6842     6833       -9     

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