Add 'user' postfix
Handle pending users in User.get_or_create_inactive

Previously, pending user objects were assumed to exist. Now, if an
active Enrollment is found (and an Actor is not), new pending
user-objects will be created.
Handle existing users when creating pending user
Add comment on a known bug (caused by Focus)
Add clarifying comment for User.is_inactive field
Retrieve the actor object, not user object
Create inactive pending user on registration if applicable

Since pending users theoretically could exist in focus.Enrollment
without having been created in our User DB, we should account for this
and create an inactive pending user object if applicable.
Set forening to null in API if pending user
Implement get_parent for pending members
Implement get_children for pending members
Implement is_household_member for pending members
Filter on active enrollments in ActorProxy init