* SsoHandler: remove inheritance from BaseHandler
* Simplify the flow for SSO UIA
We don't need to do all the magic for mapping users when we are doing UIA, so
let's factor that out.
This was broken in #8801 when abstracting code shared with OIDC.
After this change both SAML and OIDC have a concept of
grandfathering users, but with different implementations.