Filter out private projects from the API

[KasperBrandt]

Test plan

GIVEN that a project has been set as a private project
WHEN a user that doesn't have edit rights to the project requests the project in the API
THEN the project (and all its related objects) should not appear in the API

GIVEN that a project has been set as a private project
WHEN a user that has edit rights to the project requests the project in the API
THEN the project (and all its related objects) should appear in the API

GIVEN that a project has been set as a private project
WHEN a user wants to edit the project in the editor (and thus has edit rights)
THEN the user should be able to edit the project

Issue description

Private projects should be filtered from the API, as they should not be visible anywhere. Only users that have permission to edit these projects should be able to retrieve these projects from the API.