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/policies/exercise_policy.rb

@@ -37,6 +37,10 @@ class ExercisePolicy < AdminOrAuthorPolicy
     end
   end
 
+  def programming_groups_for_exercise?
+    admin?
+  end
+
   def submit?
     everyone && @record.teacher_defined_assessment?
   end