Enables serving files and bokeh apps via panel.serve

[MarcSkovMadsen]

Adds the suggested functionality of #1938

[codecov]

Codecov Report

Merging #1939 (98945f4) into master (a60e1b2) will decrease coverage by 0.03%.
The diff coverage is 74.07%.

@@            Coverage Diff             @@
##           master    #1939      +/-   ##
==========================================
- Coverage   85.23%   85.19%   -0.04%     
==========================================
  Files         152      154       +2     
  Lines       18962    18987      +25     
==========================================
+ Hits        16162    16176      +14     
- Misses       2800     2811      +11     

Impacted Files	Coverage Δ
panel/tests/io/bk_app.py	0.00% <0.00%> (ø)
panel/tests/io/panel_app.py	0.00% <0.00%> (ø)
panel/io/server.py	67.01% <100.00%> (+1.29%)	⬆️
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/widgets/tables.py	84.60% <0.00%> (-0.03%)	⬇️
panel/tests/test_param.py	99.87% <0.00%> (-0.01%)	⬇️
panel/models/vtk.py	98.43% <0.00%> (ø)
panel/pane/plotly.py	89.14% <0.00%> (ø)
panel/tests/widgets/test_slider.py	100.00% <0.00%> (ø)
... and 6 more

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