Add options to sanitize HTML panes

[philippjfr]

Since HTML panes are a potential vector for XSS attacks we add the ability to automatically sanitize HTML panes.

[codecov]

Codecov Report

Merging #5516 (2daca18) into main (a640d70) will increase coverage by 11.09%.
The diff coverage is 89.47%.

@@             Coverage Diff             @@
##             main    #5516       +/-   ##
===========================================
+ Coverage   72.46%   83.55%   +11.09%     
===========================================
  Files         276      276               
  Lines       40233    40247       +14     
===========================================
+ Hits        29153    33627     +4474     
+ Misses      11080     6620     -4460     

Flag	Coverage Δ
ui-tests	41.45% <42.10%> (?)
unitexamples-tests	72.38% <89.47%> (-0.08%)	⬇️

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Files Changed	Coverage Δ
panel/reactive.py	80.85% <33.33%> (+0.93%)	⬆️
panel/pane/markup.py	90.04% <100.00%> (+0.25%)	⬆️
panel/tests/pane/test_markup.py	99.45% <100.00%> (+0.02%)	⬆️
panel/util/__init__.py	87.71% <100.00%> (+2.91%)	⬆️

... and 71 files with indirect coverage changes

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