Set default From address in config (#3756)

The old implementation sets default From address in mailers. It sets
the address from SMTP_FROM_ADDRESS, or notifications@localhost. The
behavior is occasionally undesired results.

In production environment, notifications@localhost is likely to be
incorrect.
In testing environment, the email address should not be varied by a
environment variable.

After appling this change,

In production environment, it will throw an exception when launching
Mastodon.
In testing environment, the address will be fixed with
notifications@localhost.

config/environments/test.rb

@@ -30,6 +30,8 @@ Rails.application.configure do
   config.action_controller.allow_forgery_protection = false
   config.action_mailer.perform_caching = false
 
+  config.action_mailer.default_options = { from: 'notifications@localhost' }
+
   # Tell Action Mailer not to deliver emails to the real world.
   # The :test delivery method accumulates sent emails in the
   # ActionMailer::Base.deliveries array.