Add JSON model and Pane

[philippjfr]

Implements #936

• Add tests
• Add reference entry

[jbednar]

Cool!

[philippjfr]

Okay, I also took this opportunity to factor out a PanelMarkupView which contains the layout optimizations and improved handling for sizing modes. The previous caching approach did not handle changes to the sizing mode well but I've now modified such that switching to a dynamic sizing mode dynamically works.

[codecov]

Codecov Report

Merging #953 into master will increase coverage by 0.05%.
The diff coverage is 98.94%.

@@            Coverage Diff            @@
##           master    #953      +/-   ##
=========================================
+ Coverage   85.05%   85.1%   +0.05%     
=========================================
  Files          96      96              
  Lines       10637   10684      +47     
=========================================
+ Hits         9047    9093      +46     
- Misses       1590    1591       +1

Impacted Files	Coverage Δ
panel/tests/test_interact.py	100% <100%> (ø)	⬆️
panel/pane/markup.py	95.95% <100%> (+0.46%)	⬆️
panel/pane/__init__.py	100% <100%> (ø)	⬆️
panel/models/__init__.py	100% <100%> (ø)	⬆️
panel/tests/test_io.py	100% <100%> (ø)	⬆️
panel/pane/base.py	87.2% <100%> (+0.54%)	⬆️
panel/tests/test_layout.py	100% <100%> (ø)	⬆️
panel/models/markup.py	100% <100%> (ø)	⬆️
panel/tests/pane/test_markup.py	99.2% <96.42%> (-0.8%)	⬇️

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