No system events for access level changes

Summary

When a user's access level to a project is changed, there is no system hook. I think that a user_add_to_team event should be fired, but maybe it should be a separate event which includes the old access level as well.

Steps to reproduce

Set up a system hook and add a user to a team/group. Then change the access level of that user.

Expected behavior

Two events, one for the addition and another for the access level change (add, remove, modify, expire, etc).

Actual behavior

Only one event for the initial addition.