Add new CentralNotice cookie config var

This is to support https://gerrit.wikimedia.org/r/#/c/144853

After that is deployed, we can remove the old
$wgNoticeCookieShortExpiry and $wgNoticeCookieLongExpiry vars.

Change-Id: I9439ea6d8079742c8d9e4af10554635253df80f1
Merge "Add new CentralNotice cookie config var"