Allow nested parameterized objects on ReactiveHTML

[philippjfr]

This means that ReactiveHTML objects can now declare parameters of type ClassSelector(class_=Parameterized) which are automatically reflected as DataModels on the JS side and automatically synced.

[codecov]

Codecov Report

Merging #2533 (e32933c) into master (d27ffec) will increase coverage by 0.23%.
The diff coverage is 63.01%.

@@            Coverage Diff             @@
##           master    #2533      +/-   ##
==========================================
+ Coverage   83.00%   83.23%   +0.23%     
==========================================
  Files         184      184              
  Lines       22721    23226     +505     
==========================================
+ Hits        18859    19332     +473     
- Misses       3862     3894      +32     

Impacted Files	Coverage Δ
panel/models/reactive_html.py	79.20% <0.00%> (ø)
panel/links.py	78.48% <25.00%> (-1.42%)	⬇️
panel/reactive.py	74.56% <66.66%> (-0.81%)	⬇️
panel/layout/tabs.py	86.45% <100.00%> (+0.14%)	⬆️
panel/tests/layout/test_tabs.py	100.00% <100.00%> (ø)
panel/tests/test_reactive.py	99.06% <100.00%> (ø)
panel/tests/test_server.py	100.00% <100.00%> (ø)
panel/tests/pane/test_echart.py	37.93% <0.00%> (-1.36%)	⬇️
panel/pane/alert.py	100.00% <0.00%> (ø)
panel/tests/test_docs.py	100.00% <0.00%> (ø)
... and 95 more

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