Use Namespace#full_path instead of #path where appropriate

• Replaces Namespace#path with Namespace#full_path in a bunch of places
• Replace "#{project.namespace.path}/#{project.path}" by project.full_path in a bunch of places
• Replace <namespace,project>.<path,to_param> with <namespace,project> where appropriate since Rails will automatically call to_param, and namespace.path wouldn't cut it anyway because we need namespace.full_path (or namespace.to_param) for nesting reasons
• Update Project#reference_pattern to support nested namespaces, which means GFM references now work for projects in nested namespaces too!
• Tweak the regex for nested namespaces

/cc @dzaporozhets