Add HoloViz logos as ChatMessage avatars

[ahuang11]

Makes it more convenient to just use user="hvplot" or user="lumen" without manually setting avatar.

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 40.27%. Comparing base (c89841a) to head (1691a8c).

Files	Patch %	Lines
panel/chat/message.py	76.92%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6348       +/-   ##
===========================================
- Coverage   82.92%   40.27%   -42.66%     
===========================================
  Files         313      313               
  Lines       45834    45842        +8     
===========================================
- Hits        38007    18462    -19545     
- Misses       7827    27380    +19553     

Flag	Coverage Δ
ui-tests	40.09% <76.92%> (-0.01%)	⬇️
unitexamples-tests	22.23% <61.53%> (-48.90%)	⬇️

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[MarcSkovMadsen]

Should all the avatars not be available from cdn?

1. we are in control
2. loads faster?

[ahuang11]

That sounds good to me; I don't have experience uploading to CDN / using CDN resources though. Is it documented somewhere?

[ahuang11]