Add optional "no last name auth" setting to API

Description

Add optional setting to Django (read from EXTERNAL_SETTING_) to disallow authentication via last name.

EXTERNAL_SETTING_LAST_NAME_AUTH_DISABLED (default to False).

Further details

This does not involve any changes to the model (i.e. last name still stored as part of authentication model).

Task list

Acceptance criteria

• Setting (defaults to off) to turn off last name authentication flow

Links/references