Use Let's Encrypt for certs on staging

[humphd]

What would you like to be added:

When we ship to staging (and eventually production), we'll need proper certs that aren't self-signed. We can use Let's Encrypt for free. We'll need to do this automatically when we push to staging.

Why would you like this to be added:

We'll need to run our app over HTTPS when not in a dev setup.

[manekenpix]

2 articles that I think could be useful for adding Let's Encrypt:
https://medium.com/@yash.kulshrestha/using-lets-encrypt-with-express-e069c7abe625
https://flaviocopes.com/express-letsencrypt-ssl/

[manekenpix]

I think @miggs125 wants to try this.

[humphd]

Assigned! I spoke to @ctyler and he suggested we might want to start with some manual work to just generate and then copy/paste certs over.

BTW, I've filed #530 for the domains we'll need to get this working. Let's Encrypt needs public domains to exist.