Add skeleton for perspective

[fiskus]

Show skeleton beneath <perspective-viewer /> When perspective data is loaded, but <perspective-viewer /> isn't rendered yet

[codecov[bot]]

Codecov Report

Merging #2779 (d32093e) into master (f421dc0) will decrease coverage by 0.00%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2779      +/-   ##
==========================================
- Coverage   41.10%   41.09%   -0.01%     
==========================================
  Files         561      561              
  Lines       25721    25727       +6     
  Branches     3542     3543       +1     
==========================================
  Hits        10572    10572              
- Misses      14364    14370       +6     
  Partials      785      785              

Flag	Coverage Δ
api-python	90.71% <ø> (ø)
catalog	13.17% <0.00%> (-0.01%)	:arrow_down:
lambda	87.68% <ø> (ø)

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Impacted Files	Coverage Δ
...ents/Preview/renderers/Perspective/Perspective.tsx	0.00% <0.00%> (ø)

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[fiskus]

As I remember, there is still white surface behind table because delay before rendering happened after Perspective rendered as a component but not delivered table rows