Some providers (e.g. Twitter) don't expose user email addresses, so it
turns out we can't use that as the sole form of identification/account
matching.
This commit introduces a new `auth_tokens` table which stores arbitrary
attributes during the sign up process. For example, when Twitter is
authenticated, a new auth token containing the user's Twitter ID will
be created. When sign up is completed with this token, that Twitter ID
will be set as an attribute on the user's account.
