Pull in administrate-field-nested_has_many

[goosys]

@nickcharlton
Would you consider pulling administrate-field-nested_has_many into the core?

For many applications, nested forms are essential, and this excellent plugin feels like something that almost everyone needs.

The concept of nested relationships appears everywhere (form,flash,errors,hint,requiredness,...).
When this functionality lives outside the core, developers often end up applying a significant amount of customization when building real applications.
In my case as well, I've had to introduce several monkey patches to make things work smoothly.

I’d appreciate it if you could consider bringing this into the core.

[goosys]

• refs Improve handling of nested resources #2364

[nickcharlton]

It's also kind of tricky to maintain outside of Administrate, because it's so entwined with Administrate itself.

The original hope was that it'd a good example of a field plugin but it's probably too complex, isn't tested well enough, etc, etc.

So yes, we should probably bring it in.