Make it possible to preview pipeline success/failed emails

Visit `/rails/mailers/notify` on your local running GitLab instance to
show a preview pipeline success emails.

spec/mailers/previews/notify_preview.rb

+class NotifyPreview < ActionMailer::Preview
+  def pipeline_success_email
+    pipeline = Ci::Pipeline.last
+    Notify.pipeline_success_email(pipeline, pipeline.user.try(:email))
+  end
+
+  def pipeline_failed_email
+    pipeline = Ci::Pipeline.last
+    Notify.pipeline_failed_email(pipeline, pipeline.user.try(:email))
+  end
+end