Allow easy overriding of admin rendering helpers.

[Adam Avramov]

The classes in question -- the ones that reside in django.contrib.admin.helpers -- play a fundamental role in rendering admin forms; yet their entire existence is woefully undocumented, and the code only references them in passing in the middle of complex view code, effectively turning a crucial part of the rendering process into a black box, and thus precluding a whole lot of customization opportunities which would otherwise be accessible to the (ostensibly) few of us who love contrib.admin and use it as a go-to framework for the more formulaic CRUD projects out there.

The one minor contribution I've made to the Django codebase so far -- namely merging add_view and change_view into changeform_view on a nice Cardiff Sunday -- unintentionally made overriding helpers slightly easier, by introducing the get_inline_formsets method, where InlineAdminFormSet could be replaced with a custom subclass without much ado. Replacing AdminForm, however, still requires copying the big blob of code that is changeform_view and modifying it in place -- hardly good OOP practice.

Adding a way to specify custom helper classes would require only trivial changes to the admin codebase, so if anyone else thinks I've got a point, let me know and I can come up with a patch/PR.

[Tim Graham]

I am in favor of trying to remove these helpers as much as possible and move functionality (where/if it makes sense) into model form/formset, etc. It's also nice not to make everything (these classes) a public API so we can more easily make changes without worrying about backwards compatibility. Maybe if you can enumerate some use cases for customizing these classes we can work on a plan for how to make this easier.