Configure Rails/I18nLocaleAssignment cop to scan all Ruby files

This cop scans only the tests files by default, but we prefer to scan all
application Ruby files, so when a developer uses the class method
`I18n.locale=`, the cop will embrace using the method
`I18n.with_locale` instead. By doing this way, the cop will help
developers to avoid unexpected translation errors.

Quoting the Rails 6 guides:
> I18n.locale can leak into subsequent requests served by the same
thread/process if it is not consistently set in every controller. For
example executing I18n.locale = :es in one POST requests will have
effects for all later requests to controllers that don't set the locale,
but only in that particular thread/process. For that reason, instead of
I18n.locale = you can use I18n.with_locale which does not have this
leak issue.

Now we enabled the cop for all application Ruby files; we have to
remove the assignments at the controller level to set the request
locale. As Rails 6 guides suggest [1], we can use the `around_action`
controller callback to set each request locale without breaking the
rule.

This cop will warn CONSUL developers when using `I18n.locale`
assignment embracing them to use the `I18n.with_locale`instead.

[1] https://guides.rubyonrails.org/i18n.html#managing-the-locale-across-requests

app/controllers/subscriptions_controller.rb

@@ -1,5 +1,6 @@
 class SubscriptionsController < ApplicationController
-  before_action :set_user, :set_user_locale
+  before_action :set_user
+  around_action :set_user_locale
   skip_authorization_check
 
   def edit
@@ -29,10 +30,10 @@ class SubscriptionsController < ApplicationController
       [:email_on_comment, :email_on_comment_reply, :email_on_direct_message, :email_digest, :newsletter]
     end
 
-    def set_user_locale
+    def set_user_locale(&action)
       if params[:locale].blank?
-        I18n.locale = I18n.available_locales.find { |locale| locale == @user.locale&.to_sym }
-        session[:locale] = I18n.locale
+        session[:locale] = I18n.available_locales.find { |locale| locale == @user.locale&.to_sym }
       end
+      I18n.with_locale(session[:locale], &action)
     end
 end