Support alternate authentication providers

[ldanielburr]

Currently, Mantissa depends upon axiom.userbase.LoginSystem for user authentication and registration. LoginSystem implements IRealm and ICredentialsChecker, using LoginAccount objects to verify the user's credentials.

It would be great if Mantissa could allow users to register and authenticate via third-party auth providers, e.g., Facebook, Stack Exchange, GitHub, et cetera. This could be made possible by allowing an application developer to supply their own IRealm and ICredentialsChecker implementations to Mantissa as part of the site configuration.

It probably would require changing LoginAccount to deal with, for example, OAuth tokens instead of plain-text passwords, which would break every Mantissa app, so maybe specifying some kind of ILoginAccount interface, and allowing that implementation to be pluggable would be necessary to maintain compatibility.