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

• 343

• 342

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

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

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/django-cms/djangocms-versioning/pull/344?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms) | Coverage Δ | | |---|---|---| | [djangocms\_versioning/cms\_config.py](https://app.codecov.io/gh/django-cms/djangocms-versioning/pull/344?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms#diff-ZGphbmdvY21zX3ZlcnNpb25pbmcvY21zX2NvbmZpZy5weQ==) | `80.36% <66.66%> (-0.46%)` | :arrow_down: |

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[marksweb]

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