Rename getEmailForPasswordReset to support duplicate emails

[mikewuu]

Suggestion

Rename getEmailForPasswordReset -> getIdentifierForPasswordReset, and email -> identifier in password_resets table.

Why

The default implementation for DatabaseTokenRepository assumes that every user has a unique email. I have a custom UserProvider where that isn't the case.

I can implement a work-around where getEmailForPasswordReset returns a unique key, but it isn't immediately obvious why that's required without diving into the source code.

Other

• It looks like getEmailForPasswordReset is only used by DatabaseTokenRepository as an identifier. PasswordBroker uses the user retrieved via the provider directly.

• The default CanResetPassword implementation to use $this->email as the identifier can remain unchanged.

[mikewuu]

Seems traits such as ResetPassword expect the method to return an email. This makes renaming the method less ideal, but having separate method (and field) for the id would still be great.