Fix copy folder being broken after django-mptt removal

[protoroto]

Description

After django-mptt removal, copying folders is broken because insert_at method is not there anymore.
I've added a regression test that should show the error, and then a fix for it that make the test pass.

Related resources

• Copy folder is broken after mptt removal #1392

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Merging #1393 (a9856c4) into master (83e209f) will increase coverage by 0.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1393      +/-   ##
==========================================
+ Coverage   74.16%   74.78%   +0.61%     
==========================================
  Files          75       75              
  Lines        3445     3442       -3     
  Branches      553      553              
==========================================
+ Hits         2555     2574      +19     
+ Misses        720      698      -22     
  Partials      170      170              

Impacted Files	Coverage Δ
filer/admin/folderadmin.py	75.18% <100.00%> (+2.50%)	⬆️

... and 1 file with indirect coverage changes

[fsbraun]

Thank you! Well spotted and excellent work!