We would like to know which kind of authentication backends the server
supports.
This is information you can get from /login, but not in a way easily
parseable by API apps (e.g. the Zulip mobile apps).
Previously, api_fetch_api_key would not give clear error messages if
password auth was disabled or the user's realm had been deactivated;
additionally, the account disabled error stopped triggering when we
moved the active account check into the auth decorators.