config: fix DynamicSmtpSettingsInterceptor warning again

This warning re-appeared when running mailer tests:

```
DISABLE_SPRING=1 bin/rspec spec/mailers/administration_mailer_spec.rb
```

It is now fixed properly, in a way recommanded by the documentation.

lib/action_mailer/dynamic_smtp_settings_interceptor.rb

@@ -0,0 +1,22 @@
+# Note: this class is instanciated when being added as an interceptor
+# during the app initialization.
+#
+# If you edit this file in development env, you will need to restart
+# the app to see the changes.
+
+class DynamicSmtpSettingsInterceptor
+  def self.delivering_email(message)
+    if ENV['SENDINBLUE_BALANCING'] == 'enabled'
+      if rand(0..99) < ENV['SENDINBLUE_BALANCING_VALUE'].to_i
+        message.delivery_method.settings = {
+          user_name: ENV['SENDINBLUE_USER_NAME'],
+          password: ENV['SENDINBLUE_SMTP_KEY'],
+          address: 'smtp-relay.sendinblue.com',
+          domain: 'smtp-relay.sendinblue.com',
+          port: '587',
+          authentication: :cram_md5
+        }
+      end
+    end
+  end
+end