Allow configuring the logout page

[philippjfr]

Instead of immediately directing a user back to the login page this PR ensures that the /logout endpoint serves a page that confirms logging out and allows the user to click a button to log back in. It also makes this template configurable and adds documentation.

Additionally this PR fixes some issues with redirects, particularly when a prefix is provided.

Fixes #5492

[codecov]

Codecov Report

Merging #5514 (0a36506) into main (4496012) will increase coverage by 11.10%.
The diff coverage is 89.13%.

@@             Coverage Diff             @@
##             main    #5514       +/-   ##
===========================================
+ Coverage   72.50%   83.61%   +11.10%     
===========================================
  Files         276      276               
  Lines       40199    40233       +34     
===========================================
+ Hits        29147    33641     +4494     
+ Misses      11052     6592     -4460     

Flag	Coverage Δ
ui-tests	41.46% <89.13%> (?)
unitexamples-tests	72.46% <17.39%> (-0.05%)	⬇️

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Files Changed	Coverage Δ
panel/io/server.py	75.41% <0.00%> (+9.46%)	⬆️
panel/command/serve.py	40.22% <75.00%> (+24.63%)	⬆️
panel/auth.py	47.02% <85.71%> (+10.64%)	⬆️
panel/io/resources.py	87.28% <100.00%> (+0.02%)	⬆️
panel/tests/ui/test_auth.py	100.00% <100.00%> (+61.53%)	⬆️

... and 66 files with indirect coverage changes

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