Do not let process admins manage users

[mrcasals]

🎩 What? Why?

Process admins should not be able to manage admins. No link appears in the sidebar. Trying to access the section via URL redirects the user to the admin dashboard.

📌 Related Issues

• Fixes The Process Admin shouldn't see the Administrators list #703

📋 Subtasks

None

📷 Screenshots (optional)

[mention-bot]

@mrcasals, thanks for your PR! By analyzing the history of the files in this pull request, we identified @oriolgual and @josepjaume to be potential reviewers.

[codecov-io]

Codecov Report

Merging #729 into master will increase coverage by <.01%.

@@            Coverage Diff             @@
##           master     #729      +/-   ##
==========================================
+ Coverage   97.13%   97.13%   +<.01%     
==========================================
  Files         339      339              
  Lines        5404     5406       +2     
==========================================
+ Hits         5249     5251       +2     
  Misses        155      155

Impacted Files	Coverage Δ
.../app/controllers/decidim/admin/users_controller.rb	90.47% <100%> (ø)	✅
...dim/admin/abilities/participatory_process_admin.rb	100% <100%> (ø)	✅
...n/app/models/decidim/admin/abilities/admin_user.rb	100% <100%> (ø)	✅

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