Add a message explaining Let's Encrypt requirements

[clochix]

In order to sign the certificate, Let's Encrypt must be able to connect to the server on port 80 using the provided domain name.

The signing step often fails because this requirement is not fulfilled (for example because port 80 is filtered by firewall or not enabled in NAT). We should:

• display an explanation message
• maybe find a way to check this before trying to sign (with an external service or maybe an option of the acme tool?)

[clochix]

Closing, as the Node.js version of Cozy is now deprecated.