perf(solidus_admin): Remove created_at order

The created_at column does not have an index and there will be a tmp table created to order the result set. This will cause a delay on huge data sets.
Move the order statement into the search scopes to sort each scope individually. It is also faster to sort only one column instead of two. Only the scopes that are filtering already completed orders will sort by completed_at.
This change reduced the loading time of 8M orders from 18sec to 836ms.

Author: sascha-karnatz

admin/app/controllers/solidus_admin/orders_controller.rb

@@ -5,15 +5,15 @@ class OrdersController < SolidusAdmin::BaseController
     include Spree::Core::ControllerHelpers::StrongParameters
     include SolidusAdmin::ControllerHelpers::Search
 
-    search_scope(:completed, default: true) { _1.complete }
-    search_scope(:canceled) { _1.canceled }
-    search_scope(:returned) { _1.with_state(:returned) }
-    search_scope(:in_progress) { _1.with_state([:cart] + _1.checkout_step_names) }
-    search_scope(:all) { _1 }
+    search_scope(:completed, default: true) { _1.complete.order(completed_at: :desc) }
+    search_scope(:canceled) { _1.canceled.order(completed_at: :desc) }
+    search_scope(:returned) { _1.with_state(:returned).order(completed_at: :desc) }
+    search_scope(:in_progress) { _1.with_state([:cart] + _1.checkout_step_names).order(id: :desc) }
+    search_scope(:all) { _1.order(id: :desc) }
 
     def index
       orders = apply_search_to(
-        Spree::Order.order(created_at: :desc, id: :desc),
+        Spree::Order,
         param: :q,
         distinct: false
       )