Fix model relations to ProjectUser

The model should use a singular name. Also the user should only be referenced at
Entry and not added twice to the db.
Add a ProjectUserSerializer to list users of Entries
User is now a dict and the user_abbr isn't anymore contained directly in the
entry
Add add a related_name for ProjectUser project ForeignKey

This will allow to easier reference the ProjectUsers of a Project.
Add a ProjectUserFilter class

Currently is can be filtered by user_abbr only.
Add a ProjectUsersListAPIView
Add url mapping for new ProjectUsersListAPIView
Add hyperlink to user in ProjectDetailSerializer
Add users to Project angular ressource