[3.x] Change $page.user to $page.auth.user

[SamuelMwangiW]

This PR is proposed to nest the shared user prop under auth key as proposed in #323.

Since this is a breaking change, am targeting the next major release #323 (comment).

If approved, the docs and upgrade guide will also need to be updated.

No hard feelings on closure

[jessarcher]

I haven't tested this yet, but I've bumped into the issue when building a /users/{user} page, so I'm happy to see it 👍

[SamuelMwangiW]

Thanks Taylor