Allow email change

[eric-burel]

Notes:

• we don't know the current user email (one-way encrypted) and thus can't display it => there is a chance that users changes their email... for the same email again (they might have also forgotten which email they used initially). We should consider this ok, and could tell the user "actually you already used this email, that's fine".
• if an account already exists for the new email, this means we might need reconciliation, see #302. We could consider the most recent response as the canonical one.

This means both this feature and #302 are dependent on also creating an interface that shows user multiple response and letting them delete one or maybe "fuse" them (like keeping the most recent response for each field).

[SachaG]

if an account already exists for the new email, this means we might need reconciliation

That seems hard to do, because we need to make sure they do own the new email before proceeding with any data changes… Have people been asking for this? It might be easier to handle it on a case by case basis for now, or maybe code a simple script in surveyadmin to do it?

[eric-burel]

I was supposing that if the person is logged in, they own the email, but that's not completely true ideally we should indeed revalidate the current email.

What led me to think about this feature is that @LeaVerou was not sure whether she was connected using an email. I've added a message in the account profile that states whether you are logged in with email or as guest.

But since I can't display the email (one-way encrypted) it can still be a bit unsettling: people can't know which email they used to log in.