[bugfix] Fix slice width calculation in dashboard

[kristw]

My previous fix (#5839) was incorrect.
It only works for slices that spans 1-2 columns, but compute less width than it should be for the slices that span >2 columns.

The previous formula did not take GRID_GUTTER_SIZE into account. A slice with n columns also contains n-1 gutter.

The formula for width is updated to

widthMultiple * columnWidth + (widthMultiple - 1) * GRID_GUTTER_SIZE - CHART_MARGIN

@john-bodley @graceguo-supercat

[graceguo-supercat]

LGTM

[codecov-io]

Codecov Report

Merging #5867 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5867      +/-   ##
==========================================
+ Coverage   63.73%   63.79%   +0.05%     
==========================================
  Files         374      374              
  Lines       23325    23462     +137     
  Branches     2607     2607              
==========================================
+ Hits        14867    14968     +101     
- Misses       8445     8481      +36     
  Partials       13       13

Impacted Files	Coverage Δ
...ashboard/components/gridComponents/ChartHolder.jsx	93.18% <ø> (ø)	⬆️
superset/utils.py	89.53% <0%> (+0.5%)	⬆️
superset/cli.py	56.36% <0%> (+2.55%)	⬆️

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[williaster]

🤦‍♂️ thanks for fixing this