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

app/helpers/statistics_helper.rb

@@ -44,6 +44,7 @@ module StatisticsHelper
         key: 'programming_groups',
         name: t('activerecord.models.programming_group.other'),
         data: ProgrammingGroup.count,
+        url: programming_groups_path,
       },
       {
         key: 'currently_active',