Add PDF attachments to workshop emails

[pablojimeno]

Summary

• allow admins to attach one PDF to workshop broadcast emails
• preserve the uploaded PDF through preview, back-to-edit, send-test, and confirm-send
• persist the attachment on send history and include it in outbound mail

Testing

• bin/rails test test/models/workshop_email_draft_test.rb test/controllers/admin/workshop_emails_controller_test.rb test/mailers/workshop_email_broadcast_mailer_test.rb
• bin/brakeman -q -f json