Support admin dashboard in multi-process deployments

[philippjfr]

Fixes #3811

[codecov]

Codecov Report

Merging #3812 (df306a7) into master (a4ff6ed) will increase coverage by 0.00%.
The diff coverage is 25.71%.

@@           Coverage Diff           @@
##           master    #3812   +/-   ##
=======================================
  Coverage   82.97%   82.98%           
=======================================
  Files         221      221           
  Lines       32230    32242   +12     
=======================================
+ Hits        26744    26755   +11     
- Misses       5486     5487    +1     

Flag	Coverage Δ
ui-tests	34.15% <17.14%> (+0.01%)	⬆️
unitexamples-tests	75.46% <25.71%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/io/admin.py	0.00% <0.00%> (ø)
panel/command/serve.py	35.34% <33.33%> (+0.15%)	⬆️
panel/io/server.py	74.95% <70.00%> (-0.10%)	⬇️
panel/io/state.py	70.93% <100.00%> (+0.05%)	⬆️
panel/util.py	87.54% <0.00%> (+0.37%)	⬆️
panel/tests/util.py	87.12% <0.00%> (+1.98%)	⬆️

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