Add ability to define authorization callback

[philippjfr]

Currently our authentication simply authorizes users based on whether they have been given permissions in the OAuth provider configuration. Often times this is controlled by IT or is otherwise difficult to manage so its often easier to grant permissions to use the OAuth provider freely but then restrict usage in the application itself. If you want to do this you currently have to validate the pn.state.user or pn.state.user_info yourself and then modify your application based on whether the user is authorized or not.

To make this simpler we introduce a pn.config.authorize_callback which is given the user_info dictionary and returns True or False depending on whether the user should be authorized.

[codecov]

Codecov Report

Merging #3777 (1032197) into master (70fa6fb) will decrease coverage by 49.62%.
The diff coverage is 20.68%.

@@             Coverage Diff             @@
##           master    #3777       +/-   ##
===========================================
- Coverage   83.44%   33.82%   -49.63%     
===========================================
  Files         214      214               
  Lines       31426    31430        +4     
===========================================
- Hits        26224    10631    -15593     
- Misses       5202    20799    +15597     

Flag	Coverage Δ
ui-tests	33.82% <20.68%> (-0.03%)	⬇️
unitexamples-tests	?

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Impacted Files	Coverage Δ
panel/command/serve.py	15.00% <8.33%> (-22.12%)	⬇️
panel/io/server.py	22.56% <10.00%> (-54.04%)	⬇️
panel/auth.py	39.20% <25.00%> (-0.15%)	⬇️
panel/config.py	45.87% <100.00%> (-14.35%)	⬇️
panel/io/logging.py	100.00% <100.00%> (ø)
panel/__main__.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/katex.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/mathjax.py	0.00% <0.00%> (-100.00%)	⬇️
panel/models/perspective.py	0.00% <0.00%> (-100.00%)	⬇️
panel/pane/vtk/synchronizable_deserializer.py	0.00% <0.00%> (-97.59%)	⬇️
... and 157 more

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