[dev] fix redux devtools

[williaster]

This PR fixes redux dev tools which broke due to the webpack 4 upgrade.

The process.node. NODE_ENV variable is no longer defined, so I define the new WEBPACK_MODE variable and use its value to set redux dev tools or not.

@kristw @mistercrunch @xtinec @michellethomas @graceguo-supercat

[mistercrunch]

Oh nice! I noticed this was down a little while ago but never got to fixing it. Thanks for doing this.

[mistercrunch]

LGTM

[codecov-io]

Codecov Report

Merging #5855 into master will increase coverage by 0.09%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5855      +/-   ##
==========================================
+ Coverage   63.75%   63.84%   +0.09%     
==========================================
  Files         374      374              
  Lines       23320    23427     +107     
  Branches     2608     2638      +30     
==========================================
+ Hits        14867    14958      +91     
- Misses       8440     8455      +15     
- Partials       13       14       +1

Impacted Files	Coverage Δ
superset/assets/src/reduxUtils.js	75.4% <0%> (ø)	⬆️
.../src/explore/components/AdhocMetricEditPopover.jsx	75.91% <0%> (-0.79%)	⬇️
superset/assets/src/SqlLab/actions.js	72.06% <0%> (+0.81%)	⬆️
...rset/assets/src/SqlLab/components/TableElement.jsx	95.23% <0%> (+1.19%)	⬆️
...uperset/assets/src/SqlLab/components/SqlEditor.jsx	80.95% <0%> (+2.85%)	⬆️
superset/assets/src/utils/common.js	65.09% <0%> (+3.55%)	⬆️
...src/explore/components/controls/SpatialControl.jsx	46.95% <0%> (+14.01%)	⬆️
superset/assets/src/common.js	30.76% <0%> (+30.76%)	⬆️

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