Improve rendering of ReactiveHTML literal children

[philippjfr]

Children which are declared as literals are now synced via the DataModel and no longer trigger a full rerender.

[codecov]

Codecov Report

Merging #2383 (6df7ac0) into master (02ea38b) will decrease coverage by 0.31%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master    #2383      +/-   ##
==========================================
- Coverage   83.55%   83.24%   -0.32%     
==========================================
  Files         185      185              
  Lines       22529    22098     -431     
==========================================
- Hits        18825    18396     -429     
+ Misses       3704     3702       -2     

Impacted Files	Coverage Δ
panel/reactive.py	76.11% <73.33%> (-0.25%)	⬇️
panel/models/reactive_html.py	80.53% <100.00%> (+0.13%)	⬆️
panel/auth.py	36.21% <0.00%> (-3.30%)	⬇️
panel/config.py	56.12% <0.00%> (-3.07%)	⬇️
panel/pane/ipywidget.py	47.05% <0.00%> (-2.95%)	⬇️
panel/io/reload.py	63.95% <0.00%> (-2.72%)	⬇️
panel/tests/template/test_manual.py	59.13% <0.00%> (-2.48%)	⬇️
panel/pane/plot.py	55.61% <0.00%> (-2.14%)	⬇️
panel/io/state.py	61.05% <0.00%> (-2.14%)	⬇️
panel/pane/streamz.py	65.71% <0.00%> (-1.86%)	⬇️
... and 89 more

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