Rename 'site ssl' to 'site ssl-verify'

`ee site ssl` command is causing confusion among new v4 users. They either assume it will add SSL to a non-SSL site or they think it will be required to run every time after any SSL site creation.

Hence to clarify what `ee site ssl` does, it is better if we rename it to `ee site ssl-verify`. If the current command does both verify SSL when using the DNS method and renew if it's present, we can create another command `ee site ssl-renew` and call the same function internally.

Also, it would be great if we would be able to track if DNS challenge verification is pending on a site. We can add this info in `site_meta` and only allow `ee site ssl-verify` if DNS verification is pending.