Make Membership require uniqueness of association, add missing specs
Add PersonPresenter
Make PeopleController#show use PersonPresenter
Add memberships to routes
Complete implementation of AddingAPersonToAProject

* make it receive and pass the view_context
* make it able to command the controller
* implement #expose_form
Disable ViewingAnEntry, it is not used.
Override to_partial_path for User

* Internally, we have a "User", for the data layer and for
	authentication / authorization.
* But for the the domain, we want it to be a "Person".
Moving the memberships association declarations to User model

* WIP: move it back to the appropriate context, refactor the context in
  such a way that makes it look more natural
Remove unused view files for user
Make people_controller handle users#show route
Implement MembershipsController, which uses AddingAPersonToAProject
Add missing partials used by PersonPresenter