Add way to update user email

[SachaG]

This might be a little bit tricky because of our anonymous log-in flow, but it would be useful to add a way for users to update their emails, or at least for an admin to do it manually.

[eric-burel]

As admins we can retrieve a user document if provided the email (we need to encode it), and then we could change that for the encoded email. But to keep the process secure we need to authenticate the user, so they have to at least send us an email from their current email. If the current email doesn't exist anymore the account is technically just lost.
If people are reaching out via email, we could craft something in surveyadmin to retrieve the user document from a given email and encode the new email, maybe keep the old one as a safety net (eg "my account was actually hacked let me rollback to the older mail")

[SachaG]

Yeah I will do something like that for now.