Small ux improvement

[ipiv]

• ux - Heatmap checkbox visible only when possible to show heatmaps (more than 1 player added to compare).

• Grid container invalid prop "xs"

Warning: Failed prop type: The prop `xs` of `Grid` must be used on `item`.
    in WithStyles(ForwardRef(Grid)) (at PlayerComparePlayStyleCharts.tsx:132)
    in PlayerComparePlayStyleCharts (at PlayerCompareContent.tsx:34)

• handleAddPlayers state mutation to single instead of multiple - causing rerendering

[codecov[bot]]

Codecov Report

Merging #423 into master will not change coverage by %. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #423   +/-   ##
=======================================
  Coverage   61.43%   61.43%           
=======================================
  Files          91       91           
  Lines        5904     5904           
  Branches      844      844           
=======================================
  Hits         3627     3627           
  Misses       2143     2143           
  Partials      134      134           

Flag	Coverage Δ
#integration	50.69% <ø> (ø)
#server	59.70% <ø> (ø)

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