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/user.rb

@@ -111,9 +111,9 @@ class User < ApplicationRecord
 
   def self.ransackable_attributes(auth_object)
     if auth_object.present? && auth_object.admin?
-      %w[name email external_id consumer_id platform_admin]
+      %w[name email external_id consumer_id platform_admin id]
     else
-      %w[name external_id]
+      %w[name external_id id]
     end
   end
 end