OPSK-48 - fix such that people automatically get subcribed to a project when joining

required an additional callback for after_add and after_remove for groupmemberships.
this is in addition to the currrent callbacks for work_groups.
the dependency on these callbacks leads to some perculiar code, and cannot always be relied upon and I intend revisiting it.
OPSK-48 uncoupled subscriptions out of person and into a module

Seek::Subscriptions::PersonProjectSubscriptions holds the code for people subscriptions and simplfied the
code and little and removed duplication
removed redundant incorrect integration test

logic of the test was wrong, and was only passing due to the way the callbacks
were implemented.
removed duplicated userless_people method and some refactoring

the scope not_registered already preformed the same task