feat: Prefetch page content version objects for faster page tree

[fsbraun]

Description

To improve responsiveness of the page tree for large sites (in terms of pages), this PR adds a prefetch_related to the admin query sets and evaluates prefetched version objects where available.

This, for example, removes the N+1 issue when getting the state indicators. With a prefetched page content query set evaluating the state indicator does not require a db query.

Note: Currently, the django CMS core does not respect the admin get_queryset method which would contain the prefetch_related rule. This is fixed by https://github.com/django-cms/django-cms/pull/7956

Merging of https://github.com/django-cms/django-cms/pull/7956 is required to have a test for this PR.

Related resources

• https://github.com/django-cms/django-cms/pull/7956 is required for this PR to have an effect

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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.