SAK-25877 Allow custom logout URL for container authenticated users.

This allows container authenticated users to be sent to one logout URL and internally authenticated users to be sent to another one. This is useful where your container authentication needs to perform some cleanup.

git-svn-id: https://source.sakaiproject.org/svn/login/trunk@308435 66ffb92e-73f9-0310-93c1-f5514f145a0a