Add ComponentResourceHandler to server

[philippjfr]

This PR implements a so called ComponentResourceHandler that is always added to the server. The ComponentResourceHandler resolves paths relative to some module, i.e. it resolves patterns like <endpoint>/<module>/<class>/<attribute>/<path_to_resource>. Internally the resource handler then resolves the path to the module that was referenced, the class that was referenced and finally the attribute on that class. If and only if the requested resource is listed in that attribute will it resolve the path_to_resource and serve it. This means that ReactiveHTML and Template resources that reference local files can now be resolved correctly without having to be bundled into the Panel bundle itself.

Fixes #3241
Fixes #1787

Partially addresses #2903

• Add tests
• Fix security issues (you can only access static files which were explicitly referenced)

[codecov]

Codecov Report

Merging #3284 (b80f01f) into master (0c4408b) will increase coverage by 0.01%.
The diff coverage is 79.00%.

@@            Coverage Diff             @@
##           master    #3284      +/-   ##
==========================================
+ Coverage   83.29%   83.30%   +0.01%     
==========================================
  Files         195      195              
  Lines       26166    26325     +159     
==========================================
+ Hits        21795    21931     +136     
- Misses       4371     4394      +23     

Impacted Files	Coverage Δ
panel/template/base.py	76.83% <56.52%> (-1.61%)	⬇️
panel/io/server.py	78.41% <64.70%> (-1.79%)	⬇️
panel/io/resources.py	87.17% <85.71%> (+4.06%)	⬆️
panel/command/serve.py	37.89% <100.00%> (+0.62%)	⬆️
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/util.py	85.09% <100.00%> (ø)
panel/widgets/slider.py	84.16% <0.00%> (+0.03%)	⬆️

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[maximlt]

Would it make sense to show in the console logs what files are served?

[philippjfr]

Would it make sense to show in the console logs what files are served?

Don't think so, the StaticFileHandler doesn't do so and failed requests are logged.