The SsoSecurityConfigurer that gets added when a user has a custom
WebSecurityConfigurer with @EnableOAuth2Sso is quite opinionated, and
this is preventing users from custimizing the exception handling in the
customized UI security. This change makes it less opinionated, using
request matchers to configure the default instead of ovewriting the
single authentication entry point.
Also adds an entry point responding with a 401 for XHR clients (just like
the vanilla HTTP Basic auth).
Fixes gh-4629
cccc3867eb