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)

Users usually don't read our provided example config.php, so we should
include some settings in the admin settings. 

\cc @danimo