add mail_view for email previews

https://github.com/basecamp/mail_view

Rails 4.1 [has mail previews built in](http://edgeguides.rubyonrails.org/4_1_release_notes.html#action-mailer-previews), but [there's a bug](https://github.com/rails/rails/issues/14435) with inline attachments. Will be easy to convert once that's fixed.