Admin validation doesn't reflect ModelAdmin's extensibility

[Honza Král]

If you choose to extend ModelAdmin in a non standard way - for example overriding get_changelist, Django still validates it using django.contrib.admin.validation which has no clue on what options are valid for your ModelAdmin class.

By design if you validate something that can be extended/changed/substituted for something it must be either optional (not run whenever you try to access your admin in debug mode) or extensible itself (moved to the classes in question - ModelAdmin, ChangeList etc).

[Russell Keith-Magee]

An example, because it took me a couple of seconds to catch on - imagine you overwrote get_changelist(), and made it use the value in the list_display property of ModelAdmin. However, your revised get_changelist() doesn't use the same format as the stock list_display (e.g., it expects a list of classes, not a list of strings). The ModelAdmin validation will raise an error because list_display doesn't validate to ModelAdmin's default behavior, even though that behaviour isn't necessarily being used.

This might just be a matter of making validate() a method on ModelAdmin that can be overridden like anything else.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Honza Král]

Code can be found here:
​https://github.com/HonzaKral/django/compare/master...ticket-12674

[Russell Keith-Magee]

I've reviewed this; there's a couple of minor things that I've flagged in the inline comments, but for the most part it looks good.

[Honza Kral <honza.kral@…>]

In 4ad1eb1c14b629cf5bcfd253ed40e875f1bddd47:

Fixed #12674 -- provide a way to override admin validation

Moved admin validation code to classes and have those be class
attributes to the ModelAdmin classes.