Improved handling of ReactiveHTML resources

[philippjfr]

• Deduplicate ReactiveHTML resources
• Add example using external resources

[codecov]

Codecov Report

Merging #2397 (e2b8411) into master (483d000) will decrease coverage by 0.05%.
The diff coverage is 29.16%.

@@            Coverage Diff             @@
##           master    #2397      +/-   ##
==========================================
- Coverage   83.62%   83.56%   -0.06%     
==========================================
  Files         185      185              
  Lines       22601    22619      +18     
==========================================
+ Hits        18899    18901       +2     
- Misses       3702     3718      +16     

Impacted Files	Coverage Δ
panel/io/resources.py	75.74% <29.16%> (-5.24%)	⬇️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️

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[nghenzi]

Great Work ! This is awesome !!! it will improve a lot the visual aspects of the apps