This change disallows creation of users possessing the same email.
If a user attempts to create an account with an email address
that's already used - whether it belongs to an active account or not -
he will be notified of unsuccessful account creation. If he attempts to
log in using an email address belonging to an inactive account,
he will be presented with a link allowing him to re-send the
verification email. Attempting to register with an email address
belonging to an existing account triggers a password reset email.
Change-Id: Iefd8c3bef00ecb1dd9e8504594607aa0dca7d82e
9d13c649a2