Verify and show warning if admin custom view isn't properly extended

[darklow]

Since Django 1.9+ every admin custom view must be wrapped in each_context(request). Verify and show warning it is not done properly. Related to #563.
Docs:

• https://docs.djangoproject.com/en/1.10/ref/contrib/admin/#django.contrib.admin.AdminSite.each_context
• https://docs.djangoproject.com/en/1.10/ref/contrib/admin/#django.contrib.admin.ModelAdmin.get_urls