Add 'remember me' option to login forms

[jbroadway]

To extend the session expiry so they don't have to log in again for some time. Default expiry is 30 days, so this feature would only need to be enabled if session_duration was adjusted (for example, if it was set to 0 to expire at the end of their browser session).

Add remember_login_checkbox = On setting to Users app settings form. If session_duration < 2592000 then show the checkbox. If $_POST['remember_me'] is set, set session cookie duration to 2592000 instead of whatever it's set in conf/config.php.