Bundle .map files during build

[philippjfr]

Various debuggers and browser consoles look for sourcemap files which were previously not bundled. This raised errors in the JS console which would be distracting and cause users to think there was something wrong. During the bundling step we now simply try to make a request to the CDN to check if there's a .map file that we should bundle alongside the CSS and JS files.

[codecov]

Codecov Report

Merging #2591 (35e04ea) into master (396d814) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2591      +/-   ##
==========================================
- Coverage   83.18%   83.14%   -0.05%     
==========================================
  Files         184      184              
  Lines       23254    23263       +9     
==========================================
- Hits        19343    19341       -2     
- Misses       3911     3922      +11     

Impacted Files	Coverage Δ
panel/compiler.py	6.66% <0.00%> (-0.34%)	⬇️
panel/io/reload.py	66.29% <0.00%> (-2.25%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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