Add BuildSetting to hide Social Login

[langleyd]

The current login screen hides the inputs section (username, password, sso button) if username/password fields are not enabled and there is at least 1 SSO flow enabled.

This may be a suitable behaviour for some login screens. But for others it is preferable to:

1. Not show username/password
2. Have exactly 1 SSO flow
3. Show a single "Log In" button that launches the SSO flow rather than showing the single SSO flow on the Social login, which looks marginally more awkward.