Bundle external JS dependencies for custom models

[philippjfr]

During build time this PR would pull all external resources specified on Model.__javascript__ definitions and bundle them into the dist folder, which means when resources='server' they could be served locally.

The downside to this approach is that this grows the Python distribution by about 9 MB and this is only set to grow as more models are added. However this is the only approach that will allow these models to work in an air-gapped environment.

• Switch __javascript__ dynamically depending on the current resources setting

@mattpap @MarcSkovMadsen @jbednar @xavArtley Opinions welcome here

[jlstevens]

I think at this stage of the project it is a good idea. That said, I think we should decide on a size limit and put it in the tests...e.g a 15MB limit for now and fail if we grow past that. Whether automated or not, I think we need to keep an eye on the total size to make sure it doesn't explode.

Of course, when it makes sense things should be split up into separate packages/extensions...

[codecov]

Codecov Report

Merging #1651 into master will decrease coverage by 0.30%.
The diff coverage is 62.05%.

@@            Coverage Diff             @@
##           master    #1651      +/-   ##
==========================================
- Coverage   85.41%   85.10%   -0.31%     
==========================================
  Files         148      148              
  Lines       16840    16944     +104     
==========================================
+ Hits        14384    14421      +37     
- Misses       2456     2523      +67     

Impacted Files	Coverage Δ
panel/tests/test_server.py	100.00% <ø> (ø)
setup.py	0.00% <0.00%> (ø)
panel/io/notebook.py	57.98% <12.50%> (-1.16%)	⬇️
panel/compiler.py	11.53% <16.39%> (+6.88%)	⬆️
panel/io/resources.py	79.72% <81.48%> (-12.34%)	⬇️
panel/models/ace.py	95.23% <83.33%> (-4.77%)	⬇️
panel/models/echarts.py	92.30% <83.33%> (-7.70%)	⬇️
panel/models/plotly.py	95.83% <83.33%> (-4.17%)	⬇️
panel/models/vega.py	91.66% <83.33%> (-8.34%)	⬇️
panel/models/vtk.py	98.38% <83.33%> (-1.62%)	⬇️
... and 15 more

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[philippjfr]

So this will not work for all external dependencies, in particular if a dependency expects other files to be present alongside the main js/css files. As an example katex expects various woff font files to be present. So for now I'd only enable this for a subset of models.