Add WP_HTTPS constant to force whether HTTPS is enabled or disabled

[westonruter]

As mentioned in WordPress Slack:

Similarly to how there is a WP_HOME and WP_SITEURL constants which force the home/siteurl values regardless of what there is in the DB. We could also have a WP_HTTPS constant that does the same for the HTTPS checkbox that appears on the general settings screen. If that constant is defined then the checkbox would not be shown. So if they want to turn off HTTPS after accidentally turning it on, they could add define( 'WP_HTTPS', false ); to their wp-config.php.

This would allow sites to prevent users from turning off HTTPS to great peril, and it would also allow users to turn off HTTPS if they locked themselves out of the admin due to a bad certificate (and they can't figure out how to force their browser to accept it).