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/application_controller.rb

@@ -11,7 +11,7 @@ class ApplicationController < ActionController::Base
   before_action :authenticate_http_basic, if: :http_basic_auth_site?
 
   before_action :ensure_signup_complete
-  before_action :set_locale
+  around_action :switch_locale
   before_action :track_email_campaign
   before_action :set_return_url
 
@@ -40,14 +40,15 @@ class ApplicationController < ActionController::Base
       end
     end
 
-    def set_locale
-      I18n.locale = current_locale
+    def switch_locale(&action)
+      locale = current_locale
 
-      if current_user && current_user.locale != I18n.locale.to_s
-        current_user.update(locale: I18n.locale)
+      if current_user && current_user.locale != locale.to_s
+        current_user.update(locale: locale)
       end
 
-      session[:locale] = I18n.locale
+      session[:locale] = locale
+      I18n.with_locale(locale, &action)
     end
 
     def current_locale