Fix exclude_fields_in_copy with DeferringRelatedManager object

[elhussienalmasri]

Fixes #11715

After	Before

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[lb-]

@elhussienalmasri are you able to add unit tests for this change. The tests should fail before this change and pass after it. So that we don't break this again in the future.

[elhussienalmasri]

sure @lb- , it a bit complicated because its require DeferringRelatedManager object , but I will try to find similar unit test , and I will add unit test for this PR asap.

[elhussienalmasri]

I added @lb- the unit test , and I make migration for "RelatedManager" ( MultipleChooserPanel) object manually because when I tried to automatically run migrations some error occurs , so I had to make it manually ,so if you know how to make it automatically without any error, please let me know and I will update this PR , also I just added one unit test so if you have other suggestions about unit tests please let me know.

[elhussienalmasri]

when I run migration before I add any code this error occurs editor = editor_settings[name] KeyError: 'custom' , and after
this error occurred:
,
may it because I added two " MultipleChooserPanel" but I want to run migration automatically and solve the first error.
I just said that to take into consideration when review this PR, by the way I only want t add " MultipleChooserPanel" to
exclude object " PageWithExcludedCopyField" , may if I can run migration automatically , may I find solution to this issue,
so any idea how to run migration automatically.
I tested this issues again and it exist before this PR, the first issue "editor" in the same page cause this issue. and the second issue may be solved, by replace the "related_name" to one of them, this make conflict, and cause this issue. but I don ,t test this solution.