Automatically sign in the person when they confirm their email address

Fixes #1029

There's a security issue we're accepting here. If a person mistakenly
requests a password change to a valid but unintended email address
then the recipient could take over their account in the time between
them sending the request and noticing they'd updated their email
incorrectly.

We're accepting this risk because it seems very unlikely and your
account here isn't exactly your bank account.