Provide a views relationship from {project_release_nodes} to {node}

Thanks to #539676: Expose {project_release_supported_versions} data to views it's now possible to define a relationship from projects to releases associated with that project. This lets you have a view of project nodes, but you can pull in some release-specific fields for each project. However, that relationship has to use {project_release_nodes} as the base table, since we need to JOIN on {project_release_nodes}.pid. But, that means you can't get to all the node properties of the release nodes like {node}.created, etc. For that, we need to be able to add a relationship from {project_release_nodes}.nid to {node}.nid.