Implement GridStack layout

[philippjfr]

A GridStack layout is largely equivalent to a GridSpec however it also has the ability to interactively resize and drag around individual components in the grid.

[codecov]

Codecov Report

Merging #2375 (5b0b253) into master (415798d) will decrease coverage by 0.20%.
The diff coverage is 8.19%.

@@            Coverage Diff             @@
##           master    #2375      +/-   ##
==========================================
- Coverage   83.76%   83.55%   -0.21%     
==========================================
  Files         184      185       +1     
  Lines       22472    22529      +57     
==========================================
+ Hits        18824    18825       +1     
- Misses       3648     3704      +56     

Impacted Files	Coverage Δ
panel/config.py	59.19% <ø> (ø)
panel/layout/gridstack.py	0.00% <0.00%> (ø)
panel/layout/grid.py	75.07% <50.00%> (ø)
panel/models/reactive_html.py	80.40% <75.00%> (-0.98%)	⬇️
panel/reactive.py	76.36% <100.00%> (ø)

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