fix: PageContent extension's copy_relations method not called

[fsbraun]

Description

This PR fixes #342 and is based on #343.

CMS extensions have a copy method. It also takes care of calling copy_relations.

djangocms-versioning needs not recreate this method where it can miss the call of copy_relations but instead can use the original method instead.

Related resources

• Fix 342 hook to copy relations #343
• PageContentExtension should offer hook copy_relations #342

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

Patch coverage: 66.66% and project coverage change: -0.04% ⚠️

Comparison is base (6ac73e8) 90.90% compared to head (671fc1c) 90.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
- Coverage   90.90%   90.86%   -0.04%     
==========================================
  Files          72       72              
  Lines        2530     2530              
  Branches      356      356              
==========================================
- Hits         2300     2299       -1     
  Misses        170      170              
- Partials       60       61       +1     

Files Changed	Coverage Δ
djangocms_versioning/cms_config.py	80.36% <66.66%> (-0.46%)	⬇️

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[marksweb]

I've not ran it, but this makes sense 👍

[marksweb]

This needs an update as there's conflicts with the main branch.