Add `PolymorphicInlineModelAdmin.get_child_inlines()`

[piranna]

The same as PolymorphicParentModelAdmin.get_child_models() allows to dynamically get the child models with base_model.__subclasses__(), I would like to have PolymorphicInlineModelAdmin.get_child_inlines() method that I can overwrite to get the inlines subclasses, and don't need to set them explicitly. This would allows me to have a cleaner code, and also prevent some circular references.

[j-antunes]

Hi! Thanks for opening an issue! Please feel free to open a PR with a possible solution!

[piranna]

I have already done an implementation, just only not fully sure if it's needed at all, once I get sure I'll do a PR. By the moment, I have at f075aab a fix to properly notify with a server side 500 error when there are no configured child models, instead of a non-sense 403 error.

[j-antunes]

Sounds good!

[piranna]

You have the PR at #582, you can review and comment it. I think it works properly, although would like to test it a bit more, specially I would ensure inlines are working properly because I'm having problems with them, I think it's about config on my side instead of a bug but want to ensure it.