Add internal title for exercise

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

db/migrate/20231013194635_add_internal_title_to_exercises.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class AddInternalTitleToExercises < ActiveRecord::Migration[7.0]
+  def change
+    add_column :exercises, :internal_title, :string
+    add_index :exercises, :internal_title, using: :gin, opclass: :gin_trgm_ops
+  end
+end