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/models/programming_group_membership.rb

@@ -12,4 +12,8 @@ class ProgrammingGroupMembership < ApplicationRecord
       errors.add(:base, :already_exists, id_with_type: user.id_with_type)
     end
   end
+
+  def self.ransackable_associations(_auth_object = nil)
+    %w[user]
+  end
 end