Add default help user logo and help_text to ChatFeed

[ahuang11]

In many of the panel-chat-examples, I have to do something like:

chat_interface = pn.chat.ChatInterface()
chat_interface.send("Instructions for users", user="System")
chat_interface.serialize()[1:]

Which can be a tad tedious when repeatedly done; this PR adds a convenient way to include a help message, and exclude users from serialization. Additionally, it makes the distinction between a "help" message vs a "system" message (which presumably could be a system prompt)

chat_interface = pn.chat.ChatInterface(help_text="Instructions for users")
chat_interface.serialize()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (075b5ae) 84.19% compared to head (5f405d2) 82.65%.
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6311      +/-   ##
==========================================
- Coverage   84.19%   82.65%   -1.55%     
==========================================
  Files         301      305       +4     
  Lines       45200    45456     +256     
==========================================
- Hits        38058    37570     -488     
- Misses       7142     7886     +744     

Flag	Coverage Δ
ui-tests	38.54% <20.00%> (-2.19%)	⬇️
unitexamples-tests	71.68% <100.00%> (-0.15%)	⬇️

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