Adds a read/unread state to notification portlet

Most (All?) of this code is a copy of the hide action.

Unlike the hide action, this might not be hidden after read.  This is also permanent unlike hide which has an expiration date.  This is meant to eventually be persisted to a notification store.  Over in this public repo: https://github.com/UW-Madison-DoIT/NotificationPortlet-UWOverlay , we're spinning up a local notification store.  Check out some web services over there!

This is just the start of read/unread action.  Some UI work to follow.

By default this is turned off and this won't change any existing implementations.