Consistently resolve component resources using relative paths

[philippjfr]

Followup for #3284

This PR cleans up some of the crufty bits that were in the original ComponentResourceHandler PR. Specifically all URLs for custom component resources are now expressed relative to the component. This means we are no longer encoding absolute paths in the URL (and indeed we no longer accept absolute paths either). Additionally this cleans up the code a bit ensuring that everywhere that generates and consumes component URLs uses the same functions to do so.

[codecov]

Codecov Report

Merging #3303 (466ee90) into master (914ca6e) will increase coverage by 0.00%.
The diff coverage is 76.82%.

@@           Coverage Diff           @@
##           master    #3303   +/-   ##
=======================================
  Coverage   83.35%   83.36%           
=======================================
  Files         196      196           
  Lines       26485    26486    +1     
=======================================
+ Hits        22076    22079    +3     
+ Misses       4409     4407    -2     

Impacted Files	Coverage Δ
panel/template/base.py	77.09% <68.88%> (+0.21%)	⬆️
panel/io/server.py	78.41% <72.72%> (+0.04%)	⬆️
panel/io/resources.py	86.77% <89.47%> (-0.42%)	⬇️
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/util.py	85.15% <100.00%> (+0.05%)	⬆️

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