Create data model to store account associations

[miketheman]

Create a new table to store per-user account associations, with a reference to the user account

Store things like:

• the service name (eg github)
• external user id (string, since could be anything)
• external username (eg miketheman)
• tokens from provider (deferring for now, since we might not ever need them, preferring just-in-time user verifications)
• extra metadata (unstructured, as we learn more we can evolve the data model