Add option for chain.pem file for custom SSL

Currently, for custom SSL, we only ask for two files from the user as input `.crt` and `.pem` file.

We should also ask for an optional `.chain.pem` file which is needed for OCSP stapling. Custom SSL should run without this file, however if the user has included it, we should keep this file at `/opt/easyengine/certs/name.chain.pem`. Nginx proxy will then automatically add OCSP stapling config for that site.