fix: Grouper admin raised AttributeError when used outside the admin views

[fsbraun]

Description

This PR adds fallback methods to GrouperModelAdmin to allow for fallback methods if not all grouping fields are set. This allows to use GrouperModelAdmin outside the admin views, e.g., as a target for the AutocompleteSelect widget.

It makes pages searchable by adding search fields. Also, the page's str method is updated for search fields: This way pages are consistently represented in the admin by (str(page)) in the form of "My page title (/path/to/page)" in the current or a fallback language.

Finally, a type hint error is fixed.

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Checklist

• I have opened this pull request against develop-4
• 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 the channel #pr-reviews on our Discord Server to find a “pr review buddy” who is going to review my pull request.