Refactor some logic from LoginRestServlet into AuthHandler

I'm going to need some more flexibility in handling login types in password
auth providers, so as a first step, move some stuff from LoginRestServlet into
AuthHandler.

In particular, we pass everything other than SAML, JWT and token logins down to
the AuthHandler, which now has responsibility for checking the login type and
fishing the password out of the login dictionary, as well as qualifying the
user_id if need be. Ideally SAML, JWT and token would go that way too, but
there's no real need for it right now and I'm trying to minimise impact.

This commit *should* be non-functional.