Introduce a Skeleton component

[willdurand]

Fixes #607
Fixes #608

Supports #379

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This is a very simple Skeleton component that should fill the place where it is added. We can iterate if we need to add constraints (like range and minWidth props) but it's only the beginning of the long journey to fix #379.

The second commit refactors the VersionChooser component to see the Skeleton in action:

[codecov-io]

Codecov Report

Merging #606 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #606      +/-   ##
=========================================
+ Coverage   98.16%   98.2%   +0.04%     
=========================================
  Files          42      43       +1     
  Lines        1087    1112      +25     
  Branches      243     254      +11     
=========================================
+ Hits         1067    1092      +25     
  Misses         19      19              
  Partials        1       1

Impacted Files	Coverage Δ
src/components/VersionChooser/index.tsx	100% <100%> (ø)	⬆️
src/components/VersionSelect/index.tsx	100% <100%> (ø)	⬆️
src/components/Skeleton/index.tsx	100% <100%> (ø)
src/reducers/linter.tsx	100% <0%> (ø)	⬆️

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[bobsilverberg]

This looks good to me, thanks @willdurand. Just one nit/question.

r+wc

[kumar303]

Thanks for getting this started. There are higher priority items but I agree with what you mentioned that we are investing more code debt in non-skeletonized components each day.

I would say just hold off on skeletonizing all the existing components because that will take a lot of time and I'd like to make some headway on the comments work first. However, we should try to skeletonize new components after you land this 🎉

[kumar303]

Sorry for the late review. I think some of my comments are worth following up on.

[willdurand]

I would say just hold off on skeletonizing all the existing components because that will take a lot of time and I'd like to make some headway on the comments work first. However, we should try to skeletonize new components after you land this 🎉

I thought I left such a comment somewhere.. I totally agree and that was my plan too.