Provide additional direction on resolving N+1 issues for ModelAdmin classes

[tim-schilling]

@charettes raised a great idea where the toolbar could highlight where and how a developer could resolve an N+1 error.

For example, if the admin is setup to generate an N+1 error, we would direct them to either add the relationship(s) to list_select_related or to override get_queryset() to add a call to prefetch_related.

There may be a way to also provide this for the generic class based views as well (ListModelMixin), etc. It'd be neat to do this for DRF ViewSets/Views.

This could be difficult when the view has overriden get_queryset a few times and knowing which one should be modified. Though perhaps we start with the last defined method and point the user to that.