Implement OCSP stapling when using Let's Encrypt
Without OCSP stapling, some browsers will make two requests every time they visit the webpage of the provider: one to the provider and one to Let's Encrypt for the OCSP information.
This is a privacy problem, slows down web browsing, and is unnecessary if we implement OCSP stapling in apache.
(from redmine: created on 2016-06-01, relates #6842 (closed))