Avoid lodash-merge, which is recursive and can lead to surprises

[cbeer]

fixes #3240

[codecov-commenter]

Codecov Report

Merging #3241 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3241   +/-   ##
=======================================
  Coverage   89.32%   89.33%           
=======================================
  Files         190      190           
  Lines        3252     3253    +1     
=======================================
+ Hits         2905     2906    +1     
  Misses        347      347           

Impacted Files	Coverage Δ
src/state/reducers/companionWindows.js	100.00% <100.00%> (ø)
src/state/reducers/elasticLayout.js	100.00% <100.00%> (ø)
src/state/reducers/windows.js	96.15% <100.00%> (+0.15%)	⬆️

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