fix: #7904 - manage.py cms fixtree also fixes PageUrl model

[jrief]

Description

This fixes #7904 – check for details

Checklist

No test yet. Any hint which existing test I can use as a starting point. Needs a tree and associated page and PageUrl objects.

• 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 #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[fsbraun]

@jrief Can you complete the test with some assertion that ensures the url tree is also fixed? I assume this will only require some additional assertions in the test_management.py file.