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/programming_group_policy.rb

@@ -1,11 +1,11 @@
 # frozen_string_literal: true
 
-class ProgrammingGroupPolicy < ApplicationPolicy
-  def new?
+class ProgrammingGroupPolicy < AdminOnlyPolicy
+  def create?
     everyone
   end
 
-  def create?
+  def new?
     everyone
   end