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.

.rubocop.yml

@@ -441,6 +441,9 @@ Rails/FindBy:
   Exclude:
     - "config/initializers/ahoy.rb"
 
+Rails/FindByOrAssignmentMemoization:
+  Enabled: true
+
 Rails/FindEach:
   Enabled: true