Make it easy for users to set up a secure site by automatically getting a certificate from Let's Encrypt.
~~Use something like [lego](https://github.com/xenolf/lego).~~ See comments below.
~~If a user chooses **Production, standalone** and **Secure** during the setup process, before asking for the certificate / key paths, ask if the user wants to get a new certificate from Let's Encrypt. If so, do it -- if not, prompt for certificate / key paths.~~
If a user chooses **Production, standalone**, the "Web server mode" prompt should have the options:
* Insecure (port 80)
* Secure (port 443), manual
* Secure (port 443), automated
The `[server]` config section should have a new `bool` indicating we should use `autocert` to serve the application instead of the standard server. Something like:
Autocert bool `ini:"autocert"`
If that's `true`, use the autocert pkg to handle requests.