Rename constant to avoid collision

This wasn't actually affecting us, but it was causing developer (me)
confusion: we declare the `user` const in this block, and then again in
a lower scope (the API callback).

To alleviate that confusion, I renamed the constant at this scope.

Done during but not directly related to #2023.

app/controllers/auth0_sign_in_callback.js

@@ -16,9 +16,9 @@ const AccessTokenHandler = function (req, res) {
       const refreshToken = body.refresh_token
       const idToken = body.id_token
       const accessToken = body.access_token
-      const user = await auth0.getProfile(accessToken)
+      const auth0User = await auth0.getProfile(accessToken)
 
-      const apiRequestBody = getUserInfo(user)
+      const apiRequestBody = getUserInfo(auth0User)
       const endpoint = `${config.restapi.protocol}${req.headers.host}/api/v1/users`
       const apiRequestOptions = {
         headers: {