Add CRUD operations for Programming Groups

* Correct sorting in table
* Modify page when nested in exercises
* Fix links between pages
* Link from statistics page to programming_groups/index
* Link from submission page to programming_groups/<id>
* Allow filtering for exercise ID on ProgrammingGroup#index
* Add search fields for internal and external user id on pg/index

config/routes.rb

@@ -97,9 +97,11 @@ Rails.application.routes.draw do
       post :export_external_confirm
     end
 
-    resources :programming_groups, only: %i[new create]
+    resources :programming_groups
   end
 
+  resources :programming_groups, except: %i[new create]
+
   resources :exercise_collections do
     member do
       get :statistics