Fix: bulk edit custom fields should support multiple items

[shamoon]

Proposed change

Closes #6588

Type of change

• Bug fix: non-breaking change which fixes an issue.
• New feature / Enhancement: non-breaking change which adds functionality. Please read the important note above.
• Breaking change: fix or feature that would cause existing functionality to not work as expected.
• Documentation only.
• Other. Please explain:

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have included testing coverage for new code in this PR, for backend and / or front-end changes.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.18%. Comparing base (c294440) to head (3eb584a).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6589      +/-   ##
==========================================
- Coverage   97.19%   97.18%   -0.01%     
==========================================
  Files         436      436              
  Lines       17932    17932              
  Branches     1422     1516      +94     
==========================================
- Hits        17429    17428       -1     
- Misses        503      504       +1     

Flag	Coverage Δ
backend	95.87% <ø> (-0.02%)	⬇️
frontend	98.63% <ø> (ø)

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