Make colorpickercontrol refer to default color scheme

[kristw]

This will reduce the code that point to specific color scheme and let ColorSchemeManager be the single point of contact for obtaining color scheme.
Developers can set company-specific default color scheme in common.js.

@williaster @conglei @graceguo-supercat

[codecov-io]

Codecov Report

Merging #6031 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6031      +/-   ##
==========================================
+ Coverage   64.63%   64.65%   +0.02%     
==========================================
  Files         449      449              
  Lines       23821    23817       -4     
  Branches     2639     2639              
==========================================
+ Hits        15396    15400       +4     
+ Misses       8412     8404       -8     
  Partials       13       13

Impacted Files	Coverage Δ
superset/assets/src/modules/colors.js	44% <ø> (-7.73%)	⬇️
...explore/components/controls/ColorPickerControl.jsx	96.15% <100%> (ø)	⬆️
superset/assets/src/modules/colorSchemes/airbnb.js	0% <0%> (-100%)	⬇️
superset/assets/src/modules/colorSchemes/lyft.js	0% <0%> (-100%)	⬇️
superset/connectors/sqla/models.py	81.13% <0%> (+0.75%)	⬆️
superset/db_engine_specs.py	55.7% <0%> (+0.87%)	⬆️

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