Remove mail_safe

We use letter opener to [deliver emails on the dev env](https://github.com/fs/rails-base/blob/27f2f76871a15b82725660199794f5512b9a9e63/config/environments/development.rb#L20).

And letter opener prevents email delivery from dev envs:

*Preview email in the browser instead of sending it. This means you do not need to set up email delivery in your development environment, and you no longer need to worry about accidentally sending a test email to someone else's address*