feat :star: allow sorting users using their order count

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

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