Build the saved views GraphQL APIs

Create the APIs so the frontend can interact with saved views. We'll need support for:

• Querying saved views, with filters for subscribed as well as for listing all the saved views available in a namespace
• Updating a saved view
• Deleting a saved view
• Subscribing / Unsubscribing from a saved view (i.e. adding / removing a saved view to the nav for a particular user)
• Reordering a saved view in the nav for a specific user