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

• 1392

Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] 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[bot]]

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%)	:arrow_up:

... and 1 file with indirect coverage changes