Ensure Index for Meteor.users.services.auth0.id

[xcv58]

We need make sure that user id from Auth0 is unique, a good example:
https://github.com/meteor/meteor/blob/88d43a0f16a484a5716050cb7de8066b126c7b28/packages/accounts-oauth/oauth_common.js#L17

_ensureIndex('services.' + name + '.id', {unique: 1, sparse: 1});

A better way is that change xcv58:auth0-lock logic to use accounts-oauth package.