feat: 🎸 method to fetch users on spam management dashboard (no interface yet)

[PeculiarE]

Fixes #1810

• PR is descriptively titled 📑 and links the original issue above 🔗
• tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
• code is in uniquely-named feature branch and has no merge conflicts 📁
• screenshots/GIFs are attached 📎 in case of UI updation
• ask @publiclab/mapknitter-reviewers for help, in a comment below

[gitpod-io]

[codeclimate]

Code Climate has analyzed commit b0e8d77 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #1811 (b0e8d77) into main (51e082c) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1811      +/-   ##
==========================================
+ Coverage   67.27%   67.44%   +0.17%     
==========================================
  Files          40       40              
  Lines        1314     1321       +7     
==========================================
+ Hits          884      891       +7     
  Misses        430      430              

Impacted Files	Coverage Δ
app/controllers/spam_controller.rb	100.00% <100.00%> (ø)

[jywarren]

Very nicely done, thank you @PeculiarE !!!