Determine the reference for an issue or merge request

When mentioning issues or merge requests from other projects, it is useful to have the full reference including project path.

This will allow summaries and comments to reference cross project resources easily without having to build the full URL or path

• Add Issuable#project_reference method
• Adds Issuable#project_path method

• This will also work in a group context because the raw resource contains a reference to it's own project

Closes #175 (closed)