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
 

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

app/controllers/dashboard/successful_supports_controller.rb

@@ -27,7 +27,9 @@ class Dashboard::SuccessfulSupportsController < Dashboard::BaseController
     end
 
     def successful_proposal
-      @successful_proposal ||= Proposal.find_by(id: Setting["proposals.successful_proposal_id"])
+      return @successful_proposal if defined?(@successful_proposal)
+
+      @successful_proposal = Proposal.find_by(id: Setting["proposals.successful_proposal_id"])
     end
 
     def days_diff

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?