Fix struct hierarchy in graphql

Currently have several different structs for managing folders in Graphql -Namespace, ExistingGraphFolder, ValidGraphFolder, GraphFolder - need to review these and make sure all are needed and use eachother appropriately.

• Would be good to rethink how we do Graph/Namespace validation - quite a bit of duplicated code and would be ideal if the structs themselves done the validating so that we can be sure they are safe.
• fn Path in Namesapce should not need to return an option - Namespace should have validated the path
• We should store the relative path as a string within the namespace so that we know it is always validated and we don't have to replace \ with / everywhere for example.