Use AuthO ID as a secondary index in users table

[dpopp07]

Once the users model is refactored such that it stores both the AuthO ID and a RethinkDB generated ID, it would be helpful to use RethinkDB functionality for a secondary index.. This would allow the client to get a user's generated ID (for posting other data that needs to link to the user) using the AuthO ID that the client receives when a user logs in.