Reconciliate response automatically when upgrading account

Scenario:
- user log in as anonymous
- then upgrade to "passwordless" **with an existing account**

Currently we add the `anonymousId` to an array linked to the user, but don't actually update th existing responses.

Could be improved:
- if user has no response for the survey, we can associate it automatically, no problem
- if user already has responses... there is a conflict, this would mean allowing multiple response per user. Reconciliation is trickier in this case => we could add an email asking user for their id and do it on-demand?