Set `Auto-Submitted: auto-generated` header to emails

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20305

changelogs/unreleased/add-auto-submited-header.yml

+---
+title: Set Auto-Submitted header to mails
+merge_request:
+author: Semyon Pupkov

config/initializers/additional_headers_interceptor.rb

+ActionMailer::Base.register_interceptor(AdditionalEmailHeadersInterceptor)

lib/additional_email_headers_interceptor.rb

+class AdditionalEmailHeadersInterceptor
+  def self.delivering_email(message)
+    message.headers(
+      'Auto-Submitted' => 'auto-generated',
+      'X-Auto-Response-Suppress' => 'All'
+    )
+  end
+end

spec/lib/additional_email_headers_interceptor_spec.rb

+require 'spec_helper'
+
+describe AdditionalEmailHeadersInterceptor do
+  it 'adds Auto-Submitted header' do
+    mail = ActionMailer::Base.mail(to: 'test@mail.com', from: 'info@mail.com', body: 'hello').deliver
+
+    expect(mail.header['To'].value).to eq('test@mail.com')
+    expect(mail.header['From'].value).to eq('info@mail.com')
+    expect(mail.header['Auto-Submitted'].value).to eq('auto-generated')
+    expect(mail.header['X-Auto-Response-Suppress'].value).to eq('All')
+  end
+end