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/models/verification/management/email.rb

@@ -11,7 +11,9 @@ class Verification::Management::Email
   delegate :username, to: :user, allow_nil: true
 
   def user
-    @user ||= User.find_by(email: email)
+    return @user if defined?(@user)
+
+    @user = User.find_by(email: email)
   end
 
   def user?