[JENKINS-15757] Ignore 'remember me' if disabled in configuration

The previous implementation of 'disable remember me' just removes the
checkbox from the UI. Users still can set the parameter using a web
browser extension, or patch the login page using a Greasemonkey script
and keep using the feature. And just after the feature is enabled, users
still might be able to use it if they loaded the login page early.

This change prevents that by checking the current configuration and, if
'remember me' is disabled, ignoring any headers set.