Serve template CSS files per Document

[philippjfr]

Previously we added the Template CSS to the global list of CSS to be served. This meant that this CSS would pollute the notebook, cause issues when switching templates since the CSS would persist and cause issues when multiple pages being served used different templates or themes.

This PR replaces the default bokeh DocHandler with a PanelDocHandler which in turn replaces bokeh Resources with PanelResources. This gives us more control over the CSS and JS files being served up.

[codecov]

Codecov Report

Merging #1479 into master will increase coverage by 0.06%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1479      +/-   ##
==========================================
+ Coverage   86.01%   86.07%   +0.06%     
==========================================
  Files         139      139              
  Lines       15512    15560      +48     
==========================================
+ Hits        13343    13394      +51     
+ Misses       2169     2166       -3     

Impacted Files	Coverage Δ
panel/template/base.py	57.78% <35.71%> (+1.85%)	⬆️
panel/io/resources.py	92.06% <94.11%> (+0.22%)	⬆️
panel/io/server.py	65.42% <100.00%> (+2.98%)	⬆️
panel/tests/test_server.py	100.00% <100.00%> (ø)

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