Fix relationship problem for illustrators <-> printings.

[plural]

Because printings may have multiple illustrators and an array field AND there is a join table, the default many_to_many implementation can't join these tables properly.

All of the loaded printings were attached to the first record before this fix and the query took 10x longer.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.21%. Comparing base (5805b84) to head (6ebd8ac).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
app/controllers/illustrators_controller.rb	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #340      +/-   ##
==========================================
- Coverage   98.27%   98.21%   -0.07%     
==========================================
  Files          79       79              
  Lines        1337     1346       +9     
  Branches      160      162       +2     
==========================================
+ Hits         1314     1322       +8     
- Misses         23       24       +1     

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