Add and apply Rails/FindByOrAssignmentMemoization rule

This rule was added in rubocop-rails 2.33.

At first, I wasn't very fond of this rule. It made the code less
readable even if it improved performace in some cases.

Then I realized that in the `Admin::MachineLearning::SettingComponent`
we were using `find_by` when we should be using `find_by!` instead, and
we detected that thanks to this rule.

So, only for that reason, I'm adding this rule, but I'm fine if we
remove it.

app/components/admin/machine_learning/setting_component.rb

@@ -8,7 +8,7 @@ class Admin::MachineLearning::SettingComponent < ApplicationComponent
   private
 
     def setting
-      @setting ||= Setting.find_by(key: "machine_learning.#{kind}")
+      @setting ||= Setting.find_by!(key: "machine_learning.#{kind}")
     end
 
     def ml_info