Allow admins in send_mail event trigger to send emails to form creator and last editor

[signebedi]

By limiting the scope for this, we ensure that this will not be implemented in the email_config.yml file but instead in the form config under a relevant send_mail event trigger. Ideally, we should support a static method, or a method which will send to the user identified in the created_by field or last_editor field of the form submission in question. These, I suppose, are also somewhat static in the sense that they will not permit any specific logic. This is a child of #284 and related to #311.