FIX: Defer UsernameSuggester logic to the auto_create_account (#52)

Core takes care of suggesting usernames during normal signup. Suggesting them here as well can cause unexpected issues (e.g. when staged users sign up).

The SAML plugin currently has a bespoke `auto_create_account` implementation, which **does** need to do its own username suggestion. Therefore this commit moves all the suggestion logic into `auto_create_account`.

An integration spec for staged-user signup is also introduced.