[MergeDups] Unify sense-reference info

[imnasnainaec]

There already exists a type (MergeTreeReference) to cover the info extracted from the draggableId of both our DragSense and SidebarDragSense. So rather than manually constructing it -- e.g.,

      draggableId={JSON.stringify({
        wordId: props.wordId,
        mergeSenseId: props.mergeSenseId,
        isSenseProtected: props.isProtectedSense,
      })}

-- add a senseRef: MergeTreeReference prop for both *DragSense components. The above then becomes simply draggableId={JSON.stringify(props.senseRef)}.

This will also allow for easily adding fields to MergeTreeReference and have them immediately available when we use

const src: MergeTreeReference = JSON.parse(res.draggableId);

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.54%. Comparing base (4d79491) to head (572d009).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3490      +/-   ##
==========================================
+ Coverage   74.71%   83.54%   +8.82%     
==========================================
  Files         285       51     -234     
  Lines       10996     5178    -5818     
  Branches     1337      632     -705     
==========================================
- Hits         8216     4326    -3890     
+ Misses       2398      692    -1706     
+ Partials      382      160     -222     

Flag	Coverage Δ
backend	83.54% <ø> (-0.18%)	⬇️
frontend	?

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[andracc]

Reviewed 6 of 8 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)