Replace "GET /user" with "GET /auth"

[zomglings]

We introduced the /auth endpoint to solve issues with our query performance on GET /user.

/auth performs much better and we have tested it in production.

We should replace GET /user handler with GET /auth handler and maybe remove the /auth endpoint.

For the extra group functionality, we can add a query parameter:
GET /user?with_groups=true.

Default can be false.