Ensure config per session state works for resources

[philippjfr]

While I recently added support for per-session state on pn.config this didn't fully work for the resource related parameters such as css_files, js_files etc. because a) they are mutable so they need to create entries on access and b) while the session handler is running state.curdoc was not set to the correct value.

[codecov]

Codecov Report

Merging #2455 (a741748) into master (fbdd1fa) will increase coverage by 0.00%.
The diff coverage is 86.48%.

@@           Coverage Diff           @@
##           master    #2455   +/-   ##
=======================================
  Coverage   83.41%   83.42%           
=======================================
  Files         183      183           
  Lines       22773    22804   +31     
=======================================
+ Hits        18997    19025   +28     
- Misses       3776     3779    +3     

Impacted Files	Coverage Δ
panel/io/server.py	61.63% <50.00%> (+0.07%)	⬆️
panel/config.py	60.21% <100.00%> (+0.49%)	⬆️
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/tests/pane/test_plotly.py	98.10% <0.00%> (-0.64%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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