Remove hardcoded default email sender

And make it configurable via `action_mailer_default_from` config.

Closes #243

test/mailers/application_mailer_test.rb

@@ -0,0 +1,17 @@
+require 'test_helper'
+
+class ApplicationMailerTest < ActiveSupport::TestCase
+  def test_reads_default_from_setting_from_config
+    assert_equal 'no-reply@registry.test', ENV['action_mailer_default_from']
+
+    mailer = Class.new(ApplicationMailer) do
+      def test
+        # Empty block to avoid template rendering
+        mail {}
+      end
+    end
+    email = mailer.test
+
+    assert_equal ['no-reply@registry.test'], email.from
+  end
+end