Support trial run that displays but does not send an email

[thashepherd]

As a developer, I'd like to be able to test changes to our email templates without sending a request (that costs money!) to mailtrap or requiring a mailtrap API key.

This issue will be resolved if I can run this app (with or without docker) and see the email that would have been sent without supplying a mailtrap API key. A great solution would generate an .html file with a timestamp in the filename and automatically open it in the browser. Feel free to repurpose (and/or rename) the existing open_in_browser argument, as if we can preview email templates then opening individual spider results in the browser becomes much less valuable.