Add warning to an admin list_view if too many queries are being used

[Jacinda Shelly]

If you use a related field in a callable on a Model or ModelAdmin, and then use that callable in list_display, Django doesn't currently have a way to automatically detect that select_related should be used and performs a query for every row in the list.

While it might be possible to do this automatically, a quick way to help with this problem would be to warn a developer with something like django.contrib.messages (if enabled) that a list_view is performing O(n) queries and that they should investigate this as a potential performance issue.