[MergeDups] Unify reducer with assumption that sidebar must have multiple senses

[imnasnainaec]

This change is 

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (6ea0515) 72.70% compared to head (0642e19) 72.75%.

Files	Patch %	Lines
...rc/goals/MergeDuplicates/Redux/MergeDupsReducer.ts	88.88%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2907      +/-   ##
==========================================
+ Coverage   72.70%   72.75%   +0.05%     
==========================================
  Files         267      267              
  Lines       10198    10192       -6     
  Branches     1192     1191       -1     
==========================================
+ Hits         7414     7415       +1     
+ Misses       2432     2429       -3     
+ Partials      352      348       -4     

Flag	Coverage Δ
backend	82.78% <ø> (ø)
frontend	63.36% <88.88%> (+0.09%)	⬆️

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[jasonleenaylor]

Reviewed 2 of 2 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)