Add internal title for exercise

The internal title is designed to provide an alternative title for teachers. It is not exposed to learners.

app/models/exercise.rb

@@ -614,7 +614,7 @@ class Exercise < ApplicationRecord
   end
 
   def self.ransackable_attributes(_auth_object = nil)
-    %w[title]
+    %w[title internal_title]
   end
 
   def self.ransackable_associations(_auth_object = nil)