[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.
[JENKINS-15757] Add tests

A new test for the existing 'Remember me' functionality that verifies that
the cookie is set, and a new test showing that, even if requested by the
user, no cookie will be set, if 'Remember me' is disabled in the security
configuration.
Merge pull request #994 from daniel-beck/dont-remember-me

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