Merge tool for person-person relationship types (#1500)

[blms]

In this PR

Per #1500:

• Make Person-Document relationship type merge logic as generic as possible through mixins and inheritance
• Reuse for both that and Person-Person relationship types

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.95%. Comparing base (8edbbac) to head (b2a8829).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1724      +/-   ##
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+ Coverage    98.93%   98.95%   +0.01%     
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  Files          245      245              
  Lines        15930    16032     +102     
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+ Hits         15761    15864     +103     
+ Misses         169      168       -1