Ensure jupyter server extension handles explicit root_dir

[philippjfr]

Fixes #4017

[codecov]

Codecov Report

Merging #4029 (12bc44a) into master (9c819d0) will increase coverage by 8.97%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4029      +/-   ##
==========================================
+ Coverage   74.65%   83.63%   +8.97%     
==========================================
  Files         223      223              
  Lines       33148    33146       -2     
==========================================
+ Hits        24748    27720    +2972     
+ Misses       8400     5426    -2974     

Flag	Coverage Δ
ui-tests	37.59% <0.00%> (?)
unitexamples-tests	74.63% <0.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
panel/io/jupyter_server_extension.py	0.00% <0.00%> (ø)
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️
panel/tests/command/test_serve.py	83.95% <0.00%> (-1.42%)	⬇️
panel/io/cache.py	82.14% <0.00%> (-0.45%)	⬇️
panel/io/state.py	69.52% <0.00%> (-0.35%)	⬇️
panel/io/server.py	74.47% <0.00%> (-0.20%)	⬇️
panel/param.py	87.22% <0.00%> (+0.15%)	⬆️
panel/pane/idom.py	28.36% <0.00%> (+0.19%)	⬆️
panel/pane/base.py	87.91% <0.00%> (+0.41%)	⬆️
... and 36 more

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