Sponsor Manager Plugin Enhancements

Primary ticket for potential future improvements to the Sponsor Manager Plugin.

1. Migrate to a Dashboard Widget.
2. Set a specific end date, up to the maximum permitted end date.
3. Add an "ignore" option that removes the Role from the visible list without expiring it. (It's unclear how this would work since it would require a new data element. Maybe use ApplicationPreference?)
4. Change the Sponsor of a role.
5. Search roles on identifier, name, or email address. (This will require some structural work since there is no underlying model for the standard filtering code to use.)
6. Allow COU Admins to view roles for Sponsors in their COU.
7. Notify sponsoree on renewal.