Improve consistency of to_references in the project

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Further to this MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8232, we should consider having a common interface for all to_reference methods across all entities.

Since the main problem lies on some of the entities having from_project and target_project, but not all of them having target_project, we could add this as part of a **options keyword arguments, or create two interfaces that use (from_project:, **keyword_options) and (from_project:, target_project:, **keyword_options) or another solution.