Reduce SQL queries to find a unsolved recommended RfC

Fixes CODEOCEAN-JQ

db/migrate/20230314084733_add_index_for_recommended_rfcs.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddIndexForRecommendedRfcs < ActiveRecord::Migration[7.0]
+  def change
+    add_index :request_for_comments, %i[exercise_id created_at], where: "(NOT solved OR solved IS NULL) AND (question IS NOT NULL AND question <> '')", name: :index_unresolved_recommended_rfcs
+  end
+end