Ensure external resources are configured correctly on save

[philippjfr]

Fixes #2367

[codecov]

Codecov Report

Merging #2452 (984a946) into master (c80db0d) will increase coverage by 0.01%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master    #2452      +/-   ##
==========================================
+ Coverage   83.39%   83.40%   +0.01%     
==========================================
  Files         182      183       +1     
  Lines       22739    22759      +20     
==========================================
+ Hits        18963    18983      +20     
  Misses       3776     3776              

Impacted Files	Coverage Δ
panel/io/save.py	59.82% <77.77%> (+2.03%)	⬆️
panel/tests/io/test_save.py	100.00% <100.00%> (ø)
panel/io/resources.py	76.23% <0.00%> (+0.49%)	⬆️

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