Merge branch 'add_user_expire_leave_event' into 'master'

Differentiate the expire from leave event

At the moment we cannot see whether a user left a project due to their
membership expiring or if they themselves opted to leave the project.
This adds a new event type that allows us to make this differentiation.
Note that is not really feasible to go back and reliably fix up the
previous events. As a result the events for previous expire removals
will remain the same however events of this nature going forward will be
correctly represented.

Closes #22611 

![Screen_Shot_2016-10-06_at_09.27.52](/uploads/e4de7bd2cdedc64a9b19e56fefa21314/Screen_Shot_2016-10-06_at_09.27.52.png)

See merge request !6717