Improve handling of server prefix

[philippjfr]

Handles relative paths to static resources in all cases including:

1. Bokeh CSS and JS files
2. Panel CSS and JS files
3. Panel model JS and CSS dependencies
4. Template CSS and JS files
5. User defined CSS and JS files

[codecov]

Codecov Report

Merging #2159 (4b6af70) into master (8710077) will increase coverage by 0.06%.
The diff coverage is 91.33%.

@@            Coverage Diff             @@
##           master    #2159      +/-   ##
==========================================
+ Coverage   83.79%   83.86%   +0.06%     
==========================================
  Files         182      182              
  Lines       21679    21762      +83     
==========================================
+ Hits        18166    18250      +84     
+ Misses       3513     3512       -1     

Impacted Files	Coverage Δ
panel/command/serve.py	16.82% <ø> (-0.73%)	⬇️
panel/template/base.py	79.68% <75.00%> (-0.75%)	⬇️
panel/io/server.py	61.77% <86.20%> (+1.71%)	⬆️
panel/io/resources.py	81.14% <100.00%> (+2.57%)	⬆️
panel/io/state.py	60.69% <100.00%> (ø)
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/reactive.py	75.95% <0.00%> (+0.28%)	⬆️

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