Add feature to remove Plus site users that have been dropped from the upstream roster

When a Plus site is launched from a main site, the roster is retrieved using NRPS (Names and Roles Provisioning Service) and all of the user accounts are created and added to the Plus site. If a student drops the course or is otherwise removed form the main site, they can be manually removed from the Plus site (i.e. its Realm).

This Jira adds two features to automate the process of removing accounts from the Plus site that are no longer in the main site. After each roster retrieval via NRPS, SakaiPlus checks is there any students who have not been in the roster for a week and deletes those students from the Realm.

If the student launches into the Plus site - they are re-added or if they re-appear in the Roster at a future time, they are re-added to the Realm - the good news is that their student activity data is retained even if they are removed from the realm. So the delete and future re-add is pretty much transparent to the student.

This Jira also adds support for two optional site properties that can be set on the Plus site to control this automatic behavior:

plus:expire_minutes (default 7 days)

plus:nrps_delay_seconds (default is 300 seconds for an instructor launch and 1800 seconds for a student launch.

The defaults for these should work pretty well. These properties do make testing much easier however.