This PR is an addition to https://github.com/octobercms/october/pull/4193
Re-using the pivotWidget's and manageWidget's models for saving form data fixes https://github.com/rainlab/translate-plugin/issues/209, where translations are not saved for the model when editing it in a relation widget.
I did some manual testing with this change and everything was working as expected: Translations are now properly saved when creating or editing related models with pivot data. Creating related models in `single` mode now works as well.
Credits to @alxy for his previous work on this issue!
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