feat :star: optimize order count in user listing page

src/oscar/apps/dashboard/users/tables.py

     active = Column(accessor='is_active')
     staff = Column(accessor='is_staff')
     date_registered = Column(accessor='date_joined')
-    num_orders = Column(accessor='orders__count', orderable=False, verbose_name=_('Number of Orders'))
+    num_orders = Column(accessor='userrecord__num_orders', default=0,
+                        orderable=False, verbose_name=_('Number of Orders'))
     actions = TemplateColumn(
         template_name='oscar/dashboard/users/user_row_actions.html',
         verbose_name=' ')

src/oscar/apps/dashboard/users/views.py

         return kwargs
 
     def get_queryset(self):
-        queryset = self.model.objects.all().order_by('-date_joined')
+        queryset = self.model.objects.select_related('userrecord').order_by('-date_joined')
         return self.apply_search(queryset)
 
     def apply_search(self, queryset):