Add a session engine extension point

Summary: Ref T7673. This is really just so I can force admin.phacility.com logout when you log out of an instance, but there are a few other things we could move here eventually, like the WILLREGISTERUSER event.

Test Plan: Logged out of an instance, got logged out of parent (see next change).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7673

Differential Revision: https://secure.phabricator.com/D15629