Custom set owner can't manage permissions for owned items #2445

Describe the bug
Issue with new feature: Entity Ownership System #2436
The option "Manage permissions on own book, chapter & pages" is still checking for the creator instead of the custom set owner.

Steps To Reproduce
Steps to reproduce the behavior:

1. Go to 'Setting' and give a user role the option to manage their own permissions
2. Create 2 users with the user role
3. Create a page with user 1
4. Change owner of page to user 2
5. See error: Still able to change permissions with user 1
6. Login with user 2 and see error: No option to change permissions with user 2

Expected behavior
After changing an owner the new owner should be able to manage the permissions for owned items instead of the creator.

Screenshots
Error for step 5

Error for step 6

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): BookStack v0.31.0
• PHP Version: 7.2.24
• Hosting Method (Nginx/Apache/Docker): Apache

Additional context
Just recently updated from v0.26 to 0.31. Thanks for the new features!