Add security section to admin settings to enable the HTTPS enforcement

Currently it only allows the admin to enable or disable the HTTPS
enforcement, but in the future it could be expanded to further options.

The HTTPS enforcement only allows the admin to enforce it, if he is
connected via HTTPS. (To prevent admins to enable it without a proper
SSL setup, which would lead to a bad user experience, as he wouldn't be
able to connect to oC anymore without changing the config.php)

Users usually don't read our provided example config.php, so we should
include some options in the admin settings. 

\cc @danimo