letsencrypt: More info on cert validity, and a delete button

Created by: JoKeyser

Hi, this PR yields more visible information of LE certificate validity, e.g. whether a certificate is revoked (via OCSP), or expired, or invalid for other reasons (such as a test certificate). That info is extracted from certbot, see here for the parsed human-readable reasons. In addition, a delete button is included, which functions independently of all other buttons. I briefly thought about revoking a certificate first before deleting it, but as it is, the user would have to do that.

Below, a comparison with/without this PR, with the same certificate status (only the order of rows in the table changes, unfortunately).

Current master: Note the missing information that the cert for "test2" subdomain is revoked; and the Revoke button is still shown. With this PR: Note the information "Revoked" for the "test2" subdomain, and missing Revoke button. Similarly, there would be "Expired on {{ expiry_date }}" or "Invalid test certificate" as other possible reasons for an invalid certifiate. Also, any certificate may be deleted (and the button order is changed to make them line up across rows).