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 django-cms/django-cms#7956

Merging of django-cms/django-cms#7956 is required to have a test for this PR.

Related resources

• fix: Refactor menus app: significant time saving (queries and cpu) django-cms#7956 is required for this PR to have an effect
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Checklist

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