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/production.rb

@@ -73,6 +73,8 @@ Rails.application.configure do
   config.action_mailer.perform_caching = false
 
   # E-mails
+  config.action_mailer.default_options = { from: ENV.fetch('SMTP_FROM_ADDRESS') }
+
   config.action_mailer.smtp_settings = {
     :port                 => ENV['SMTP_PORT'],
     :address              => ENV['SMTP_SERVER'],