Fix user session redirect.

This currently redirects people to main app routes rather than Spree
routes.

This would lead to the following 500 error:
```
NoMethodError: undefined method `new_spree_user_session_path' for #<ActionDispatch::Routing::RoutesProxy:0x0000000c9a5e00>
```

As `new_spree_user_session_path` is not defined in the main app routes.

This likely occurs in other controllers as well.