Add Rails Mailer previews

[zachmargolis]

I wanted a way to investigate #5498 (comment), so I remembered this nifty Rails feature: mailer previews!

1. go to http://localhost:3000/rails/mailers
2. click a mailer, see a preview!

lets us do cool things like check the plain-text equivalent and see in other languages

description	screenshot
list of mailers
individual preview

[aduth]

Is there any way to make these route specific? If it's anything like the CSP headers, I worry the development-specific allowances risk giving us a false sense that things are working correctly. That being said, I doubt we'll have much use for frames outside these previews 🤷

[zachmargolis]

There's an allow-from option, so I can try that

[zachmargolis]

oh wait allow-from is obsolete: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options

would you prefer ALLOWALL or SAMEORIGIN? I feel like they're roughly the same level of risk since in local development everything is from the same origin, so we can't quite list the specific pages?

[aduth]

Pardon my ignorance of the feature: Is it limited to specific environments out-of-the-box?

[zachmargolis]

Yeah, it's limited to development by default, but I'm not finding any direct documentation, only indirect documentation (ex stackoverflow for enabling it in prod which implies it's not enabled in prod, and this tutorial)

[zachmargolis]

Code climate is lagging... going to admin merge